Tahoe upgrade external drive problem

There were a few prior posts on this forum from users who couldn’t access their external drive due to the drive manufacturer’s software that they used to encrypt the drive. Lacie/Seagate, Samsung, SanDisk, etc., all provide 'drive management' software on their drives and using such 3rd party software can cause issues when macOS is updated and the 3rd party software becomes incompatible. Not sure if that's the issue here, but FYI the best practice with a new external drive (SSD/HDD, not applicable to NAS drives) is to use Disk Utility to complete erase it, format it as GUID/APFS and apply encryption if desired.

FWIW, I have a bunch of Samsung T7 SSDs ranging from 1 TB to 4 TB and they all work fine with my M4 Pro MacBook Pro running Tahoe.

Just to confirm, in Finder menu > Settings > General tab, is the box to show External Disks ticked?

Drives do fail, unfortunately. Having that happen in conjunction with a macOS update is an apparent coincidence – it's not the fault of the new OS, though when you connect the drive Spotlight will re-index it and that can sometimes exacerbate a pre-existing problem with the drive.

How old is the drive? There was a class action lawsuit against SanDisk a few years ago over failing SSDs.

For a test, try turning FileVault off in Privacy and Settings.

Doing that solved a problem here with the mounting of external drives at boot time.

Have you checked SanDisk website to see if they might a solution to fix the problem with the External SSD Drive?

System Settings >>Privacy & Security >>Security (At The Very Bottom) >> Accessories. Click on it and it will give you four choices.

Interesting that would cause the problem. I would suggest you tell Apple about it

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