External Disk Won't Mount with Apple's Big Sur Operating System

JHMoseman, I'm not sure about the error. But can you launch disk utility and then plugin or try to mount your drive causing the error? Also what kind of drive is it? Are you using any 3rd party software to manage the drive?

Paul

Did the Seagate come with any software on it that you installed to managed the drive? If so uninstall it according to the developer's instructions and boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Ok, so Disk Utility saw the disk. Can you tell how it is formatted? and/or partitioned? Also boot up in Safe Mode and does the disk mount? Did your drive come with any special software? Check with Seagate and see if they have an updated version of their software. Also does this drive have its own power supply or does it get power from USB?

Paul

You are welcome.