external drives
When I upgraded to Big Sur, external drives no longer recognized
iMac 21.5″ 4K, macOS 11.6
Question marked as
Top-ranking reply
are you using any of the manufacturer's preloaded software on the drive that is supposed to make it "mac ready"? if yes, that could be the cause of your issue. when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software.
if you are using such software, i think your options are somewhat limited.
you may want to contact the manufacturer and see if there is an update for their software.
alternatively, if you have a backup you can revert your macOS to the version of macOS that the drive was working on and transfer your data to another storage device and then erase and format the existing device before putting it back into service.
Erase and reformat a storage device in Disk Utility on Mac - Apple Support
File system formats available in Disk Utility on Mac - Apple Support