After upgrade to Big Sur iMac does not boot correctly and boots to recovery when external drive with time machine backups is connected

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After upgrade to Big Sur iMac does not boot correctly and boots to recovery when external drive with time machine backups is connected

2014 21.5” iMac

Catalina OS 10.15.7 is on external 500Gb Samsung T3 SSD (has been for years)

External 4TB Western Digital for Mac’s local backup as well as a network backup drive for other MacBooks on the network (has been working perfectly for over a year)

Saturday morning I upgraded to Big Sur and after it was done, I found the system had booted to recovery mode.

I tried everything (well almost everything as I leaned today, keep reading ) and I couldn’t get Big Sur to boot normally.

On Sunday I reinstalled from a backup from before the upgrade and guess what, it still booted to recovery.

I tried restarting without the 4TB backup drive connected and it worked (I wish I would have tried this after the upgrade to Big Sur). Once it is booted I can connect the backup drive and all works as expected.

I have made sure the Start Up disk is set properly to the T3 SSD and it still does not boot properly if the backup disk is attached. Even holding Option at power one selection the bootable drive doesn’t work.

Whenever the backup drive is attached at power up the computer seems to get confused during boot and ends up in recovery mode. This wasn't an issue before the upgrade to Big Sur, and it is still an issue after the full recovery of my Catalina backup. I have reset PRAM and SMC and it still happens. I really don't want to have to make sure my backup disk isn't plugged in every time I restart the machine.

Does anyone know what might be happening and how to fix it?

Posted on Nov 16, 2020 10:40 AM

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luckyfuso Author

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Nov 20, 2020 8:40 AM in response to luckyfuso

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.

I hope this helps someone else.

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After upgrade to Big Sur iMac does not boot correctly and boots to recovery when external drive with time machine backups is connected

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