Same issue here with a 4 TB WD My Passport on Big Sur 11.6.2
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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.