I had this happen 4 TB WD My Passport on Big Sur 11.6.2, the 1 TB partition for my Time Machine didn't come up and mounting gave the "disenter error 49218". Here's what worked for me (at least for now):
https://developer.apple.com/forums/thread/680500?answerId=676624022#676624022
"Try to mount the disk, and switch to Terminal and write sudo pkill -f fsck"
Attempted to run First-Aid on that partition.
Restarted the computer.
Time machine came back after that. No clue which of the steps may have actually fixed it. Will find out the next time that this inevitably occurs.
HeCardReadGood
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution
Post
Replies
Boosts
Views
Activity
Same issue here with a 4 TB WD My Passport on Big Sur 11.6.2