Time Machine back error regarding encryption

The assistance guide was of no use. What was important was the naming, in fact so important, that I wonder why it isn't mentioned in the instructions. It should say, in BOLD, "When re-creating a new backup on an existing disk drive once used for a Time Machine backup, you must rename the physical drive to something other than the drive name used previously for a Time Machine backup."

You see, when I "selected the drive", in Time Machine, I made the assumption that what Time Machine was doing was echoing back to me the NAMES of the AVAILABLE DISK DRIVES. Curiously that's not what it was doing. It continued to "see" an old drive volume named BACKUP. Apparently, this was not the physical disk drive named BACKUP (Why? Why? Why?). Once I erased, and initialized the physical drive named BACKUP, and gave it a new name BKUPNEW, and then added this to Time Machine as the backup drive, well by golly it worked. Why isn't Time Machine actually looking for physical drives as named by the operating system? It's beyond my logic to follow that one.

The other very curious thing is that the error message relayed none of this; not even remotely close. It was mentioning "old encryption" as the source of the problem. Encryption actually had absolutely NOTHING to do with it. The drive was never encrypted and that wasn't the source of the problem.