Can't boot from external SSD drive after Big Sur install

If the Startup Disk Control Panel does not work, you can Option-boot to Big Sur.

"Some Big Sur startup volumes don't appear in the Startup Disk Preference Pane"

https://bombich.com/kb/ccc5/macos-big-sur-known-issues

Big Sur should be able to be installed on an external drive. I would question that response from Apple.

However, I did have an issue booting after upgrading to Big Sur. My post is here: https://discussions.apple.com/thread/252047787

I placed the Big Sur installer on my internal drive, launched it, and choose an external APFS formatted SSD for the installation. On reboot, the installer remembered the external drive, correctly finished the installation, and I set the startup drive to be the external SSD in System Preferences : Startup Disk. As long as that external SSD is connected, the Mac will boot to it, and when it is removed, the Mac boots from the internal drive.

Apparently, Apple Support misadvised you.

That doesnt make sense. I installed Big Sur on an external ssd the day it came out (attached to thunderbolt 3/usbc plug). It worked. BUT it also screwed it up a bit. When restarting on the internal Catalina drive, the external big sur drive shows up as "updater" and nothing thats on the drive can be seen/mounted. BUT it does start up on it!

That is normal. Big Sur uses a signed system volume with snapshots and you actually are booted from this Update partition when in Big Sur. Older systems will only show this one partition. More @ https://eclecticlight.co/2020/06/25/big-surs-signed-system-volume-added-security-protection/

I can't explain why this works, but I was able to solve my issue.

As explained above, I am, and have been, booting form the external SSD for years with no issues. Soon after I set up my external SSD, years ago, I deleted all the files from the internal Mac HD and used that disk for non critical storage.

Upgrading my external SSD to Big Sur exposed some problem and I think it is related to the internal HD, even though I had nothing critical on that drive. Maybe there was a recovery partition on that drive I didn't know about.

After I upgraded to Big Sur, if I had my Time Machine drive attached the computer would boot to recovery mode. After trying a bunch of potential fixes, including repairing drives, the computer would start up with the picture of the folder with the question mark and sometimes finish booting.

Last night, I decided to install Big Sur on my internal HD (which did not have an OS prior to the upgrade) even though I am not booting from it. I guess the install put information on that disk that makes the computer happy and it now boots to the external SSD every time regardless of what other additional external drives are attached.

This worked for me and I hope it helps you.

I can't explain why this works, but I was able to solve my issue.

As explained above, I am, and have been, booting form the external SSD for years with no issues. Soon after I set up my external SSD, years ago, I deleted all the files from the internal Mac HD and used that disk for non critical storage.

Upgrading my external SSD to Big Sur exposed some problem and I think it is related to the internal HD, even though I had nothing critical on that drive. Maybe there was a recovery partition on that drive I didn't know about.

After I upgraded to Big Sur, if I had my Time Machine drive attached the computer would boot to recovery mode. After trying a bunch of potential fixes, including repairing drives, the computer would start up with the picture of the folder with the question mark and sometimes finish booting.

Last night, I decided to install Big Sur on my internal HD (which did not have an OS prior to the upgrade) even though I am not booting from it. I guess the install put information on that disk that makes the computer happy and it now boots to the external SSD every time regardless of what other additional external drives are attached.

This worked for me and I hope it helps you.

Same here.

Hmm. That may be “normal” but its a terrible idea. Really makes me want to sit out this “update”.

I was ninja‘d, wanted to suggest the same. A customer‘s Mac mini exhibited the same. It would not boot from the external SSD unless the TM drive was unplugged.