Just ran square into this issue today too. Brand new 8TB WD Elements drive formatted in APFS. Connected via USB 3 to a thunderbolt dock.
Error: Could not mount “Time Machine 8TB”. (com.apple.DiskManagement.disenter error 49218.)
Worked perfect as a Time Machine Disk for the last 31 days. Then abruptly stopped and will not mount.
First Aid checks showed nothing wrong with the disk.
I'm running Big Sur 11.6.2 on a 16 inch 2019 MacBook Pro.
JayHotals
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