a lot of my external drives have this error Could not mount “BUFFALO 3TB”. (com.apple.DiskManagement.disenter error 0.)

I have the exact same problem. Did you find a solution? Thanks!!

That worked! After a few weeks of cooling down (and buying another external backup disk) I reconnected the faulty backup disk once more. After about 30 minutes, the Disk maintenance app came on, told me I could no longer write on the disk and advised to reformat it. This time reformatting worked and now I have two Time Machine backup disks in alternation—some degree of trust in Apple was lost for a long time though, I fear.

So my solution above works to get the OS to let you access the drive. The problem is my drive has a corrupted catalog so there are some parts of the time machine backup that i couldn't access. The drive will have to be reformatted.

That being said there is a hidden gem of a macOS app that is a miracle worker. I've used it to recover many damaged HDD's. Even drives that GeekSquad and other computer repair companies said was a lost cause other than paying a data recovery hundreds or thousands of dollars to recover. I was able to do a deep scan using this program and recover pretty much app the files. Though some are no longer in their folder hierarchy. But not having lost thousands of photos made it well worth it.

Its called DiskDrill Pro by CleverFiles DO NOT BUY THE ONE IN THE MAC APPSTORE!!!! go to the Clever Files website and buy the Pro version. This app is GOLD if you need to recover a damaged HDD. Though I have had one, and only one HDD that it couldn't read. The icon should look like this.