How to Dual Boot and Install macOS Monterey on to an External SSD Drive with M1 Max MacBook Pro & M1

In this video I'm showing how to dual boot and install MacOS Monterey to an external SSD drive for my M1 Max MacBook Pro. This will then create a dual boot Mac system. This also works for M1 Pro MacBook Pro, Mac Mini M1, MacBook Air M1, MacBook Pro M1 and iMac M1. In fact, this dual boot for Mac tutorial will work for any Apple Silicon ARM based Mac computer. It may also work for older Intel X86 Mac computers such as Mac Pro, MacBooks, iMacs etc. with Intel processors.
In the tutorial I also show you how to format an external drive in MacOS Monterey using Disk Utility, which is absolutely necessary in order to prepare the external SSD drive so that you can install MacOS Monterey on to it.
This tutorial also shows you how to download MacOS Monterey from the Mac App Store.
For this particular setup I'm using an external Thunderbolt SSD which uses a fast NVME SSD inside it. This is one that I've put together myself, however you can use an 'off the shelf' Thunderbolt SSD. I will be showing a tutorial soon that guides you through how to build a Thunderbolt SSD.
Here's a link for some Thunderbolt SSDs on Amazon: geni.us/ThunderboltSSD
You can also use an 'off the shelf' external USB C SSD for your external boot drive. These won't be as fast as a Thunderbolt SSD but will be cheaper. A good place to start for an external USB C SSD is the Samsung T7. I will be uploading a video soon showing the 1TB T7 as an external storage drive for the M1 Max MacBook Pro.
Here's a link to the Samsung T7 1TB on Amazon: geni.us/Samsung1TBT7
Here's a link to the Samsung T7 2TB on Amazon: geni.us/Samsung2TBT7
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[@richardgilman4602]

Installed Monterey on an OWC Envoy Pro Electron external SSD. It now functions as the primary drive for my Mac mini M1. The clear and concise instructions worked flawlessly. Much thanks.

[@raosam]

Very simply and cleary explained. And you take your time, unlike the other crazy youtubers who race through things as though they have a flight to catch. Looking forward to more videos like this

[@anjirmonster]

Actually the pace was perfect. Thanks

[@DavidHarry]

Thank you 👍 Cheers, Dave.

[@steveky406]

Great vid David speed and explanation was spot on

[@Peter-jm7zt]

Great video and not boring at all, thanks for showing it. All the best regards Peter

[@flyingpig2288]

I appreciate your detailed explanation.

[@DavidHarry]

Glad it was helpful!

[@YYChinook]

Thanks for the full explanation

[@DavidHarry]

Hi Stan. You are very welcome 👍 Cheers, Dave

[@TonesAdventuresHD]

This is pretty useful if you want to test certain compatibility issues with apps and new macOS updates. Good one David!

[@DavidHarry]

Alright Tone. That’s the main reason for me and the game stuff. On Windows I’d use disk images which would give me security against installing bad software or corrupting things. I’d make a disk image of a perfect installation setup after getting all my main software installed and setup properly, then make a disk image. After that point, if I ever destabilised my Windows setup, corrupted anything, installed anything bad or got a virus, I’d just take 10 minutes to re-image the boot drive back to a perfect setup. With Mac it’s impossible to make proper disk images on any M1 Macs due to the new Secure Enclave thing and the way the SoC controls and monitors the actual drive data, you can with Intel Macs. Using Time Machine isn’t the same thing as a proper image either, so having a separate boot drive to test installs and for game stuff is about as close to keeping things as safe as using a disk image that I’ve found with M1. Plus, games and stuff can quickly fill a drive. I’m gonna be going mad for a few weeks now with videos, a lot of product stuff that I’ve been sent. I’ve got these amazing magnetic stands for the iPad Pro, almost makes the iPad look like an iMac and are awesome for desktop setups. Cheers, Dave.

[@DanielBrosman08211986]

@@DavidHarry: Do you have lots of crocks where you live?

[@andsiephil]

Thank you very much, you’ve been a great tutor , best regards.André

[@DavidHarry]

Thank you for your kind words, André, and you are very welcome. Cheers, Dave.

[@bayukris6995]

thank David its an option to upgrade SSD capacity, helpful video

[@DavidHarry]

Hi Kris. You are very welcome and I’m glad the video was of help. Cheers, Dave.

[@ponchongkong]

Thanks !! You video is so helpful

[@seart21bv]

Very helpful, thanks 🙏

[@DavidHarry]

Hi Mark. You are very welcome. Cheers, Dave.

[@andysaputra]

20:57 you can define which disk is going to be your default startup disk by going to System Preferences -> Startup Disk.

[@myp0h]

Great video.

[@DavidHarry]

Glad you enjoyed it

[@JohnSpringateMusic]

Great video David many thanks-there is an easier way to change the drive-if you go to preferences and click on Start up disk. you can select for there :)

[@DavidHarry]

Thanks, John 👍 Cheers, Dave.

[@youniv]

Thx for your informative video. Will try it in coming hours. I have two questions : 1.What happens if you just unplug the ext drive? Will it boot the internal disc without any errors? 2. Can you please make a Geekbench test for both environments?

[@porschehouse]

Thank you

[@DavidHarry]

You are very welcome. Cheers, Dave.

[@siegeINK]

This was so helpful. Thank you David!!! I got the new M1 14” 512 and have an external 1TB that I loaded as my Boot drive. Use migration assistant to transfer internal data over to external and so far so good. Easy process!!! 🙏🏼🙏🏼🙏🏼

[@MichaelAbellera]

how's the performance? is it a lot slower when swapping?

[@DavidHarry]

Hi. It's my pleasure and I'm glad it was useful. Cheers, Dave.

[@siegeINK]

@@MichaelAbellera hey Michael. So far it’s been great. My typical workload is graphics based like Adobe illustrator, playing Spotify, and transferring data to and from my Dropbox. Nothing video heavy like editing or anything ….yet. For me the speeds are great, but granted this is my newest computer switching over from a 2008 MACBOOK! Lol. Not even from a pro, but I had a 1TB SSD I replaced in that that held me down for a while until it became obsolete because of OS limitations for that machine so, for me this M1 is a dream. Not sure if that gives you a good reference on performance? Booting up from my external SSD has helped keep space free on the internal M1 (512GB) as that was beginning to fill up quick because of my linked Dropbox account and files that I kept local vs online. Excuse the lengthy response, hope it gives some kind of performance reference.

[@stephenhall3515]

This is an exemplary tutorial and only as long as it needed to be for David to efficiently and calmly demonstrate what can be daunting. It is free from too much techie talk for its own sake and 'Americanisms' which plague so many tutorials. David's real time method here (except for where fast forwarding is usual) left him open to error but this is so much better and honest than most. Also he explained why Boot 2 came up and how to get the choice panel back. David also, perhaps inadvertently, gave a tutorial within a tutorial on M1's annoyingly confusing sign in vagueness which Apple still hasn't revised. I have subscribed and expect that thousands of people will do the same for such high quality practical tutorials. Many thanks David Harry.

[@DavidHarry]

Hi Stephen. Wow, what an awesome comment, thank you. I’ll try and do some more tutorials similar to this one. I don’t always pace myself like I’ve done in this video but I am aware of people watching who’s first language isn’t English. I usually talk quite fast and am aware that my accent is also quite strong. So pacing it is definitely the better option for those watching and listening. There’s actually a few more videos I have on my list of things to do which are very specifically M1. I’m actually a long time Windows user and system builder, so as I’m learning more about Mac and ARM, I’ll make videos about the stuff that I think may be useful to others. Also. Thank you for the subscription, it’s very appreciated and massively helps my channel. Cheers, Dave.

[@giavee9186]

Thx Dav

[@DavidHarry]

It’s my pleasure and you are very welcome 👍 Cheers, Dave.

[@Ashishkumar-br4jq]

Wow great video❤ in my mac when I connect t7 it says smart status not supported even doing APFS format

[@russellwilliams846]

I really appreciated your deliberate pace on this video. Even I can do this after this detailed tutorial. Now, I'm planning to use a 4tb ext drive to load Monterey (Ihave it on the Mac Book already). If and after I do this, will I still have partitions available for the software I want to run (Pro Tools). Apologies in advance if you've shown this on another video. Thanks!

[@DavidHarry]

Hi Russell. Thanks for the kind comment and you are very welcome. You can further partition a large drive if you need dedicated audio library space or even dedicated recording/playback space for your DAW. I’ll be doing a video soon about putting together a very affordable USB C/NVMe external drive for either, an extra OS installation, or for dedicated storage. In my case this will be used with Cubase for my project and library folders, just to keep them away from the main OS drive/partition, which will also help with TimeMachine backups etc. Although, you can do pretty much anything you’d want with the drive and partitions. Cheers, Dave.

[@andreylirey7991]

Спасибо огромное, очень сильно помог))

[@EvoNijo]

Ciao Richard, complimenti per il tuo video, molto interessante. Anche io ho provato ha realizzare questo progetto, senza riuscirci, purtroppo dopo aver istallato non parte dal secondo SSD esterno.

[@hallyfella]

Great video , i have seen many video's on this subject but would like to know what difference does it make if you set the ssd to extended journaled and not apfs . My Imac is 2017 4k. thanks

[@bsf1754]

Thanks for the video. I did this test on flash memory usb3 128gb. But in the end when booting it stuck on the apple logo and then restarts automatically

[@jamskof]

i purchased a pretty cheap orico usb-c enclosure from amazon, supposedly 10gbps. i put a samsung 980 pro 1tb in there. i ran a speed test, i get about 980 read 910 write which i was happy with, i believe usb-c wont go much faster.. i decided to make it into a mac os monterey boot drive, im running after effects, cinema 4d and parallels desktop from it... and no, not all at the same time.. the booting from startup seems to take a while, then i get a bit of spinning beachball for the first 10 minutes or so, like from when it boots up, if i waited a few seconds and tried to launch an app, it would go to beachball and might hang for a minute. it also might do this whilst using some software or even just looking through folders. secondly, whilst rendering jpegs in cinema 4d, i noticed inconsistencies in the render times for each frame, sometimes it would stop rendering for a few seconds and then start again. i just feel like theres some kind of a read write issue.. maybe because the enclosure was cheap? it was only 30 australian dollars.. but it appeared to have the same spec as ones 2/3 times the price. i believe the enclosure is using realtec parts. just wondered if you had any thoughts?

[@Ashishkumar-br4jq]

May i take the back up of internal drive from second booting system?

[@KingSlayerVolt]

Any external disk disconnect issues when the Mac goes to sleep?

[@ehsanghazavi470]

Will I be able to move files from my internal ssd onto another external drive via this method?

[@joaoballa888]

The best video. One question. When booting from external ssd does the swapping memory occur with external ssd or internal ssd? Thank you so much.

[@DavidHarry]

Good question. I’m not entirely sure, although I’m guessing it’d be the internal SSD as that would already have the space allocated on it for the swap file. Happy Christmas. Cheers, Dave.

[@joselazo3145]

I get stuck in a kernel panic after I try to boot Monterrey from my external SSD. I’m using an HP p500 500gb

[@HowardBrazee]

I need to install the previous version of Monterey that didn't kill a necessary program that I have. But I don't know how to find it.

[@axxel71]

hi, frist thank you for this video I have been searching for this for ages. I have followed the steps using a 13" M1 MacBook Air and I am now at the boot menu where my internal and WD Green 1TB. When finish installation restart once then when restart twice restart with the internal drive. then i go to option restart when I select continue for the USB drive it does the spinning animation for a few seconds then just sits there, never tries to boot. I am trying to make a Sonoma external drive from a mac on Sonoma. when i select in the system setting i tray to select the external drive an put this "The version of macOS on the selected disk needs to be reinstalled". Please help me

[@volkanersoy8084]

For the last screwed-up part: Choose Startup Disk from System Preferences.

[@rajsub12]

Nice video - want to install Asahi Linux (on an NVMe M.2 - WD Black SN850, 2TB) on top of Monterey (maybe a small partition with Windows too). Looking to build docker images for a cloud native environment. Docker detect ARM and downloads base images that will not run in the Cloud (GKE, Azure or AWS). Asahi Linux has now come a long way and support hardware accelerators (and will only get better). If you ever get to doing that - would appreciate a follow up video on that too. I thought I'd put an explanation of why I need to do this as I noticed you got hassled by someone who insisted on Parallel's being enough for everyone (each to his own).

[@DavidHarry]

Hi Raj. Thanks for your comment. I’ll have to take another look at Asahi. The last time I looked I wasn’t particularly happy with certain security concerns due to the root access and the graphics for 3D wasn’t really up to much but as you’ve said that the hardware acceleration is now supported, it may be worth a good look. As you quite rightly say, there are many reasons why someone would want to dual boot and not just to replicate the same OS. As you are obviously aware, the likes of Parallels, while being good at what it does, is not the same thing and can’t be used for certain things where you’d need a genuine bootable OS, not a VM or emulator. I’m actually on the search for Android on Apple Silicon. This would likely be a VM that could run native on ARM. I’m trying to find an alternative for gaming on Apple Silicon as most routes via Parallels and Crossover are losing too much in VM, translation layers and even emulation. With Android’s ARM requirement being very close to Apple’s ARM variant for their SoCs. Android could be a good way to utilise all the 3D ability of the Apple GPU. Plus many portable games run better natively on iPad Pro than any emulated or virtualised games on Mac using Parallels and Crossover. Cheers, Dave.

[@gmanglenn1]

Is it Monterey OS compatible with Sonoma and then M3

[@entrepreneurhrthinks3661]

Mine is not able to restart properly after installation , any help ?

[@hanishrathi]

Hey David, can you help me with a issue I am facing with my mac. I updated my mac and since then it hangs at booting up bar. But works fine with safe mode. What could be the issue. I have tried 5 times to install os, wiped it clean and did it again but the issue persisted. Please help.. thanks

[@alanjackson2540]

Question...trading horses requires split second timing. If one was to do this on Mac OS from an external drive like your set up would it cause any sort of delay? Thank you.

[@kesler20219]

Note: If you have Ventura on Macintosh HD, you would have to download Monterey Installer Via USB then boot into the disk to install.

[@ivonnemartinezt1308]

Hello, I've a Macbook Pro 2019 with 250 GB internal, but I can't even install the software I need, I work on something that requires more. I would like to use my external solid drive as primary and I'm confirming whether to select AFPS or Extended Journaled. It would be to install OS X on that 1 TB SSD external device and also use it, not only to create the installation image. Do you know if I select AFPS again in that case (and GUID partition map)?

[@luismanuelmendoza789]

Can you use that external ssd in another mac?

[@tbone45040]

i'm not offered GUID Partition Map in Disk Utility nor the APFS format! i'm running Mac High Sierra 10 .13.6

[@123starthere]

Fantastic and extremely helpful thank you! Is it possible to partition the M1's internal drive and run it as a dual boot? Thank you for your time!

[@DavidHarry]

Hi, Joey. Yes, you can just partition the internal drive. I only done it this way because I needed a lot more space for the second install and what was going on it. Cheers, Dave.

[@123starthere]

@@DavidHarry Thank you once again!

[@hardlines2635]

I’m new to Macs, my question is, why would you do this instead of cloning the internal drive.

[@siddhartrockx2070]

Mac OS not showing in apps store

[@eraytezcan8160]

So U can boot windows too

[@thebozzIDW]

Great informative video, I am wondering what I might be doing wrong as I had attempted to install the latest version macOS Monterey on two separate 64 GB USB sticks, one of which was a USB-C stick, however neither device would boot. Would the use be that I need to use an SSD as the external drive?

[@Ronny.T]

Hey, I'm running into the same problem... Were you able to get it working? Did you try the SSD method?

[@seeker-of-knowledge-777]

@@Ronny.T As am I; I used a Samsung T5 as well as an SSD connected via an external enclosure. Neither of them are able to boot. When selecting the external hard drive from the boot menu, it just stalls for a couple of seconds and nothing happens.

[@vegasvideoguy]

David, this video was 100% useful. Plus, you don't have to apologize for the lengthy portions of this tutorial.

[@DavidHarry]

Glad it was helpful!

[@siegeINK]

I agree. There’s enough of us that appreciate the quality of the content that we’re willing to watch all the way through 😉

[@Ashishkumar-br4jq]

If i use a brand new ssd for booting as my mac is in recovery mode due to I delete the os mistakenly will it support in recovery mode or does it Samsung software allow me it ask for to set up first or ask for password 😢

[@slightbleeding]

Hi David, i've a managed macbook pro m1 from my company and i will use it also for me in private. Can i setup the bootdrive also on my normal private iMac computer and put it later on the macbook? I`ve no access to the app store on the company macbook to install the os...

[@DavidHarry]

Sorry, I'm not sure if it works by setting up the drive on one Mac and then using it on another, it's not something that I've tried. Cheers, Dave.

[@raosam]

I have one question though, David. Is it possible to install another OS (older one like Mojave) on an external disk and then boot into that ? I have Monterey on my main boot disk, but I want to boot ( and install) older apps on an external drive. Is it at all possible? would appreciate a revert

[@DavidHarry]

Hi. Sorry, I don't know the answer to that question as I've not tried Mojave. However, I have used two versions of OS between the internal and external storage. I think the OS installer and its installation is independent of what the OS is on the computer. As in, the OS installer looks at the drive you are installing to and sees that it does not already have an OS on it. In which case, it just goes ahead and installs to that location as there's no earlier OS on the drive that can get in the way of the install. Sorry I can't be more specific to your question about Mojave. Cheers, Dave.

[@GreginLA18]

HI David I tried to install it exactly as you did. The mac restarts and goes back to the login screen and there are folders on the samsung but it's incomplete . Not much in the folders. When i go back to the installer and select the drive it says that i can't install it on this drive but the install is incomplete and need to erase it I tried this on 2 different T7 drives. APFS on volume etc. It's monterey 12.3.1 on an M1 Macbook Pro Any suggestions?

[@entrepreneurhrthinks3661]

I have same problem

[@GreginLA18]

@@entrepreneurhrthinks3661 I bought a pluggable thunderbolt 3 drive. I will try it on that and let you know here if it works. I got it on Amazon with a 70 dollar coupon so it was like around 200 with tax for a 512GB. I know pricy but I want to see if it will work 😅

[@GreginLA18]

@@entrepreneurhrthinks3661 I just installed Monterey onto a pluggable thunderbolt 3 drive using my M1 MacBook Pro. It installed quickly and I successfully booted off that drive. Wow it’s a fast drive. I bought the 512GB one with a 70 dollar Amazon coupon.

[@techverrueckt]

On the Installation Routine after the first restart my MacBook don’t resume the installation. Its boot the system from the internal drive and thats it. I dont habe any idea whats the problem is. Can anybody help me?

[@aAatangen]

I have the same problem. Did you solve it?

[@techverrueckt]

@@aAatangen no sorry i didn’t get an answer 😣

[@aAatangen]

I just cloned my fusion drive and it works!👍

[@SCRATCHazz]

Can I have a Dual Boot for different user profile. What I want to do is that there would be ability to use the same laptop for 2 people, but on different drives. Will be there any problems with 2 Apple IDs?

[@DavidHarry]

Yes, you can setup multiple users on either drive. The external OS drive (second boot) acts the same as the internal drive. Just setup a second user as you would with the internal drive. Cheers, Dave.

[@back-seat-driver1355]

does not seem to work on macos Ventura 13.0, Any info or help on that!

[@back-seat-driver1355]

at least not on mac studio!

[@jeffgreene468]

@@back-seat-driver1355 I was able to do it on 13.2.1 on a Mac Studio. Be prepared for a LOT of reboots though!

[@back-seat-driver1355]

@@jeffgreene468 hi, thanks , but could you give me a few more words on it?

[@alexroot1980]

It doesn't work with me (I have Mac mini m1)... After the restart it prompts me to the internal ssd... (11:22).

[@andihendragunawan1844]

hi, hav you found any solution for this issue?

[@SkylarRemington]

Is anyone else getting “unable to set startup drive” (SDErrorDomain error 108)?

[@skittou]

I get that using a Samsung T5 SSD. Apparently it can be a problem when using an USB-C SSD.

[@mapzep]

Yes, I’m having the same error message with my 4TB Samsung T5 drive it’s frustrating

[@alexroot1980]

@@skittou Samsung​ t7 is usb c as well. I have checked the specs online. And... I'm having the same issue with a wd passport.

[@vegasvideoguy]

Question: When adding Windows 10 to the external boot drive, knowing one has to install Paralles. Where does Paralelles get installed, on the main/internal ssd or the external drive or both?

[@nicholasewentworth]

Both

[@nicholasewentworth]

Unless you have separate id

[@nicholasewentworth]

Because it app

[@nicholasewentworth]

Depend on your settings

[@nicholasewentworth]

Essentially it’s your choice

[@MichaelAbellera]

Mine worked well on Sandisk Extreme Portable SSD, but when I did it to my NVME DIY external SSD it is having a problem. It will finish the installation and restart, but after restarting, it will not finish the loading bar with the apple icon, no date, and etc, etc setup. it just restarts again automatically and then proceeds on loading my internal SSD. If I try booting to it, it does not proceed, noting happens... What could be the problem? :/ I successfully did it on my other external SSD, I don't know what is happening to this one. -_-

[@MichaelAbellera]

NVM, to those who encounter this type of problem. just don't let your mac sleep. press space or any letter on your keyboard repeatedly but not too much for it not to sleep until you get to the initialization screen... it is booting from the external that's why it sleeps, small problem but it could mess up your installation and you will have to redo it again.

[@editpills8467]

@@MichaelAbellera hi, im having the same problem. So what u r saying is while it is restarting i have to keep clicking on the keyboard? Im using a NAND sata ssd drive. Also, can u plz tell me which sandisk u used without any issue? Is it the 1st gen with 500 mbps speed or the new one with 2K mbps speed?

[@MichaelAbellera]

@@editpills8467 That's actually basically it, just don't let your mac sleep during the entire process. all of my OS SSDs went well with that process. My first external ssd boot was sandisk extreme portable ssd 1st gen (500 R/W), but I get bottleneck during renders as it sometimes uses your ssd, specially if you are in 8gb mac spec. that's why I opted to change it to NVME ssd on Thunderbolt 3 enclosure and now it performs nearly identical to mac's internal ssd.

[@bw162]

Good info but I don’t see the purpose as you described it. Windows and Parallels are just apps like any other on you MacBook. Why separate them out? Seems like a lot of trouble if you are in one startup disk and need to launch another app that resides on the other disk.

[@DavidHarry]

This is ideal for separating specific software which may need a lot of extra downloads that you don’t want disturbing your main setup. it’s something that’s very obvious to anyone who knows why they’d want to do it.

[@bw162]

@@DavidHarry Been using Parallels and Windows with a number of applications for many years on my Macs. They never “disturbed” anything nor have I ever heard anyone besides yourself describe it that way.

[@DavidHarry]

Like I’ve already said, for those who know what they’re doing, they will know exactly why they’d do this.

[@bw162]

@@DavidHarry The only reason you gave in your video and haven’t added one additional reason is to “not clog up your internal storage”. I guess only a genius like you would understand that scientific explanation.

[@DavidHarry]

Again, you have no clue whatsoever as to why someone wants to dual boot. Just because you’re ignorant don’t assume everyone else.

[@johntimothyescote8374]

then how will you transfer your datas from the internal to the external drive if the 2nd drive is already installed as a 2nd boot drive?

[@nicholasewentworth]

iCloud

[@nicholasewentworth]

Usb

[@nicholasewentworth]

Air drop

[@nicholasewentworth]

I’m not sure what else

[@nicholasewentworth]

Servers

[@RenatoBorocci]

Thank you for your excellent tutorial. I tried to install Monterey in my MacBook Air in an old WD SSD and it is all ok, but in a new faster SSD WD My Passport SSD 1TB with NVMe USB-C i have this message: sderrordomain 108 and the system is not readable. Have you any idea to solve the problem?

[@toshiomori8352]

Yes, I had the exact same issue with a SanDisk Extreme 1TB

[@RenatoBorocci]

When I restarted the Macbook by holding down the power button, I changed the security parameters, I redid the whole procedure and then everything worked

[@anshgautam5817]

@@RenatoBorocci Yeah the solution I wanted

[@rbellot]

I have a new Mac Studio (2202) M1 Max 32GB, macOS Monterey 12.6.1. Purchased I have a SanDisk 4TB Extreme Pro Portable SSD/formatted to APFS. I installed MacOS Monterey 12.6.1 on the SanDisk 4TB Extreme Pro Portable SSD, using my Macbook Air. When I try to boot off the Sandisk drive via my Mac Studio, I am not able or allowed to do so. When I attempt to use Startup Disk in System Preferences, I am able to select the SanDisk drive, "HOWEVER", when I press restart, I get this message..."Unable to set startup disk An error occurred while setting “Untitled” as the startup disk: The operation couldn’t be completed. (SDErrorDomain error 108.)" When I turn on my Mac Studio, while holding the power button for 30sec, I see both the Mac internal drive, and my external Sandisk drive, but I am NOT able to boot from it. Selecting the options logo puts me in recovery mode. I have already selected the Utilities Drop Down menu and selected "Startup Security Utility'', I have "Reduced Security" Checked off, "Allows any version of signed operating system software ever trusted by Apple to run" Can you tell me how to set my Mac Studio to boot off of the SanDisk 4TB Extreme Pro Portable SSD without all these roadblocks....

[@DavidHarry]

Hi. I’m not too sure but this may be a security measure so that you can’t boot from a MacOS drive that wasn’t prepared and installed on the system that you’re dual booting from. I know that pre Apple Silicon Macs could boot from drives prepared on another Mac but with Apple Silicon, Apple have introduced stronger hardware protection. Cheers, Dave.

[@nicholasewentworth]

This doesn’t even work

[@9909984984]

SDErrorDomain error 108 >>> how can solve it ? pls help me