Here's a slightly different twist on the encryption issue, which maybe someone can provide some insight to. I know this thread is a few months old, but maybe some of you are still watching it.
Device is a new 2017 15" MacBook Pro, and after days of getting it all set up I wanted to do a TM backup. I mistakenly did not format the drive as encrypted, but in a senior moment selected to "encrypt backup" from TM instead. (I have always formatted the drive as encrypted and not the backup but, now much to my regret, I did the reverse here).
Encrypting data of course takes forever, and after waiting 12 hours to be only 1/3 through 250 gigs, I called it quits to restart it all. Probably should have unmounted the drive by force quitting TM, but I instead closed the lid and thought I could pull the plug on it...the OS provides no 'nice' way of stopping the encryption. If anything, unplugging might corrupt data, but nothing physical on the drive itself...I hope!
Reformatting was kind of a bear though...after multiple failures, I had to finally do it unencrypted first, then encrypted. Ran a first aid check and no errors, so I don't think I did physical damage by unplugging it with the lid closed during encryption above. But the darn Time Machine kept wanting to encrypt the backup even when not checked and so I had to terminate again and do the entire process above over with reformatting as an encrypted drive. (Incidentally, this previously was an encrypted drive I used for TM with my previous MacBook Pro. Never a hiccup with it until this incident.)
Since it still wanted to encrypt the TM backup, I deselected the drive and then re-enabled it for TM after formatting yet again (so that's like three times now). After making sure that encryption was off, I lastly excluded everything but my shared folder (which has nothing much in it). But it still wanted to decrypt the drive...but with only 100 megs it went fast. I thought I had finally figured it out now that it was decrypted. Double checked again that backup encryption was off, and removed the exclusions. The backup seemed to go normally with the full 250 gigs...until...
The darn thing is now unencrypting the full backup, even though it was not set to encrypt it and it had already previously decrypted the partial 100 MB backup! This is making me batty. Now it's been unencrypting for 10 hours and is less than 1/4 the way though...at this rate it will be going for two more days. It's like it still thinks there was an encrypted backup to begin with, but I wiped it clean with multiple formats.
Since everything I've tried has failed, I'm letting this run out and hopefully that will be the end of it and it will then work as an unencrypted backup on an encrypted drive, but I'm not holding my breath on that. It's early evening Saturday now, so I'll find out maybe late Monday or Tuesday, unless another approach can be found.
Any inkling what is going on here? Could there be a .plist or other file somewhere that needs to be removed? I'm starting to wonder if I need to low level format the drive...haven't done that since like 1995! Or is TM somehow seeing the unique S/N of the disk or something? I am totally baffled!
Thoughts and suggestions welcome, hivemind!
Thanks!