I was able to recover mine - here's how I did it. I'm going to write out my troubleshooting process as I'm not sure what 100% fixed it.
As I read through this discussion, I was dealing with identical scenario's... my Seagate 4TB external HD, on my Macbook Pro (late-2015) running on Big Sur, wasn't mounting / not being displayed in Finder.
After failed attempts of mounting & running first aid
@Bev_Tabet_Photo suggested downloading/updating the WD Driver (https://support.wdc.com/downloads.aspx?p=294) which I did, although I don't see how this would have played a role in fixing my issue, but it is something I did.
I then reset the SMC (https://support.apple.com/en-us/HT201295)
From here, I opened Activity Monitor and quit the fsck_ext process found under the 'disk' tab
This resulted in the external hard drive being "fixed", however it was only readable, and could not copy any files to it.
I ejected the disk, restarted, reconnected the drive and opened Disk Utility. I ran a new attempt at First Aid, which took a fair amount of time (10-15 minutes)
The First Aid was successful, and I was able to write to the external drive. So far, so good.
Hope this post can manage to help others. Merry Christmas!
