MacOS Time Machine on encrypted external HDD
I've upgraded my ten-year-old MacBook Pro to a MacBook Air. The old one will go to my wife's niece as it still performs well, especially after I replaced the battery and installed an SSD to speed things up a bit. She had taken a shine to it so my upgrade makes two people happy. For my part, the difference in OS between my iMac (Mojave) and the MacBook (El Capitan) was starting to present issues, such as incompatibility between versions of Pages, Numbers etc.
Having somewhat laboriously cleaned the old MacBook of my data so that I didn't lose certain software, such as the old but still serviceable Office 2011, I ended up with a much cleaner computer that I believe will be perfect for her. It's amazing how many personal identifiers exist within the software but I'm sure that I've removed most of them as well, of course, as signing out of all the Apple services.
Next came the the job of backing up my new MacBook to the external HDD that I use for a Time Machine. I encrypted it when it was first formatted and when I tried to delete the old backup I was informed that I didn't have the necessary permissions. I therefore erased the disk and again reformatted as encrypted. Time Machine asked if I wanted to use the disk and at this point I made a mistake. It asked if I wanted the data encrypted. Because I had already encrypted the disk I didn't choose this option. When it then asked for the disk password I wasn't paying attention and entered it. Unfortunately this started a decryption process that after three hours hadn't hardly registered on the progress bar.
I abandoned this and again reformatted, this time unencrypted, and when Time Machine started this time I chose encrypted. The backup was created after about three hours and I assumed that it was encrypted on the fly. Wrong ! After the data transferred I was again in an encryption process that seemed as if it would last days rather than hours.
After a bit of internet research I found the approach that I should have taken. Format the disk as Journaled:Encrypted, select 'encrypted' when setting up the disk on Time Machine, and the data then transfers to the encrypted disk without a hitch in around two hours.
The really annoying thing is that as all this was unfolding a feeling of déjà vu came over me. Sure enough, going back through my blogs I had indeed met a similar if not quite identical problem in the past. Oh if only I had a better memory!
Perhaps after falling in the same trap twice if there is a next time I might well remember.