I also have this issue - in my case with a 2020 Macbook Air Core i7 (just pre m1)with 16GB and 1TB- I have 2 external SSDs hooked up via USB3 both of which are are formatted under APFS and used as Time Machine backup disks & supposed to alternate.
I had no issues until upgraded to Big Sur 11.2.2 48 hours ago...now neither volume mounts nor is even findable via Disk Utility (or any other method either).
I do not want to reformat and start again because of their use for restoring old document versions.
I ahem tried restarting and every other method I can think of. there is something afoot with the Big Sur External Disk recovnigitopn system...it needs a Bug Fix!!! Or at least that someone at Apple to acknowledge it exists!
Markpf4
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