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

After the Fusion Drive in my iMac (Late 2014) began to slow down, I installed Catalina on an external SSD which worked great until Big Sur.


Repeated attempts to install Big Sur on the external SSD resulted in error messages. I called Apple Support and was told that installing on an external SSD wasn't a path recognized by Big Sur.


I had no choice but to install Big Sur on the Fusion Drive which is painfully slow. I've been searching for a work around, but haven't found one.

Posted on Nov 15, 2020 7:49 AM

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Posted on Nov 16, 2020 11:38 AM

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

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14 replies

Nov 15, 2020 7:55 AM in response to Bob Grip

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.

Nov 20, 2020 8:44 AM in response to Bob Grip

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.

Nov 20, 2020 9:11 AM in response to Bob Grip

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.

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

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