Mac Mini M1 Using External SSD

Booting on external drive M1 Mac

For background, I’m Coming from 2018 Intel Mac Mini using an external Plugable Thunderbolt 3 NVMe SSD as my boot drive.  

I purchased the M1 Mac Mini in the 16gb with 256 SSD thinking I could save the $400 and keep using my external drive. Boy I did not know what apple had in store for me...

After spending hours unsuccessfully trying to use the drive, I turned to this forum for help. Like many I found help and got as far as booting into the drive but unable to clone or backup from time machine to instal all my apps, files, profile, etc. searing other forums, and piecing advice together, I was finally happily running the M1 Mac mini with my external drive as intended.  

Her is how I ended up getting it to work:

I Setup new M1 Mac Mini using internal drive. Used migration assistant to clone external intel drive from my 2018 Mac mini to the internal M1 drive. I then booted into M1 internal drive. 

Since the M1 Mac can not use a boot drive that was created from an intel Mac, I had to erase the external drive using disc utility and reformatted to APFS and GUID. You have to do this from your M1 Mac. This is a critical step, intel boot drive won’t work on M1 Mac.

Restarted M1 Mac using the new system restore start up options (hold down power button on the Mac mini until options menu loads) from there I had to change security settings to “low security”on the internal M1 drive. (If you don’t do this, migration assistant won’t work) Then instal fresh copy of Big Sur onto external drive using recovery menu within the startup options screen. (You must instal Big Sur from this menu, you can’t instal it through the App Store).

After an hour or so, the M1 Mac booted from the external drive and I was presented with language selection screen. (Be patient, system will restart several times) From there I continued setup and used migration assistance to clone internal M1 drive to the “new” external drive.

Missing any of the steps above will break the chain that allowed this to work. 

Good Luck!! 

Have you tried to make a bootable Big Sur flash installer via the Terminal, boot from it, erase the external device (not just the volume under it) as APFS GUID, then install Big Sur on the external disk? And you already allowed external booting by visiting the Recovery mode? Or is the external boot option missing there?? Disclaimer: I don't have M1 Mac mini, only Intel.

Peymon wrote:

I was using T5 Samsung and didn’t work , then I used My G technology External 1T and worked perfectly to startup externally

steps

1- hold power to get to options

2 - disk utilities and erase and partition

3 - reinstall Os from recovery.

57 min and be patient and it will work.

now I am migrating from TIme machine to new drive.

Good luck. Others have reported getting as far as you

only to have it not be bootable the next time.

I just purchased a Sandisk 1T SSD Ext

and did the following ( a bit different than last)

1- tugged ownership in get info on and off to disable

2- used a short USB c cable came with SSD

3- in disk utilities used Group Erase/ Partition

4 - did the last from Option by holding power

Down and using option and disk utilities

5 - after oration installed Bigsur from same Recovery boot

6- last 4 min took forever , but finished as I was looking at Install logs under the widows and checking errors and progress

once complete don’t touch anything

it goes and dies some edited on and off with status bar ( if boot correctly to correct disk it will be a slow status bar)

7- it bitt completely and started me as a new computer

good luck

Question.... have you been doing the installation via USB-C up to

this point (not including your new TB3)? Reason I asked is

that a I have come across a few cases where people trying to install

and boot from USB-C was a no go, but switching cables and do the

install on the USB3 "A" ports and boot worked on the Mini and the same

people also said that once they did that and switched to the USB-C

cables, the same would boot from USB-C.

Sorry some missing ( speed loss is)

since I have it working after three days of talking to Apple techs and reading

answer was always in front of me try another drive and that did it

so , Option disk choice or Startup disk in System Pref

both will do

Ars Technica's Big Sur review:

"Apple does say that Apple Silicon Macs can boot from external volumes without changing the Mac’s security settings, unlike Apple T2 Macs—this is because these security settings can now be set per volume. You can boot your internal disk with all of Apple’s security protections intact and boot in a reduced-security mode from an external disk without needing to change anything."

https://arstechnica.com/gadgets/2020/11/macos-11-0-big-sur-the-ars-technica-review/10/#h2

FWIW, just updated to macOS 11.1. The external drive I previously created at

least no shows up in boot selections when entering recovery but now has

different errors trying to boot but could be due to installing via the

pre-update Recovery system.

Too late in the day to mess with trying another install so that is a task

for tomorrow.

P.S. The 11.1 update brought back my start up chime.

Something I noticed when running "diskutil list" in terminal,

is that the internal physical drive partitions are quite different

than in the past, i.e. no EFI:

/dev/disk0 (internal):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme              500.3 GB   disk0

   1:       Apple_APFS_ISC ⁨⁩                    524.3 MB   disk0s1

   2:     Apple_APFS ⁨Container disk3⁩  494.4 GB   disk0s2

   3:      Apple_APFS_Recovery ⁨⁩             5.4 GB     disk0s3

The external drive has an EFI partition. and not the

Apple_APFS_ISC ⁨⁩ which would seem to be the boot

partition.

Yes the first time I did use external was saying can’t update

so I reformatted and reinstalled

sec time was a charm

I also couldn’t use backup the first time because of update

but sec time after reinstall was a charm

I don't think the M1 based machines will allow external booting. These new Macs can only work with the specific Big Sur System that supports the new chip.

Rosetta 2 will handle 64-bit apps that need translation.

I use an external SSD via the Thundebolt port from WD and do not have any issues. It's 2TB, so 1TB is dedicated to Time Machine, and the other 1TB is for my files, so I don't tax the SSD that came with the computer.

MyApple8MyPC wrote:

...

BTW, I tried the Startup Security Utility as shown here;

https://support.apple.com/en-us/HT208198

but there was no external drive option. ...

FWIW, I see the same thing as you on my new MacBook Air.

There is the OS security options but no option to allow

external booting.