Is it OK for Time Machine to take a long time to decrypt a freshly formatted encrypted disk?
Solution 1:
I’ve had the same question, and after a bit of research (and based on my own experience) I’ve come to some conclusions:
Encryption made by TimeMachine and Encryption made by disk utility is the SAME thing and it is done only one time.
Based on your question I think that this part of TM is poorly implemented (and NOT explained by apple), in fact when you, after encrypting the backup, selected the option to NOT encrypt it in TM the system asked you for the encryption password and done a (I guess) total decryption of all the free memory on the disk, instead of simply reformat that (and I think that TM does that to be “transparent” in that operation without having to ask to reformat the disk in an encrypted format).
THEN TO ANSWER YOUR QUESTION
“ My question still remains: Can someone tell me what is wrong with my process? I want to use the filesystem encrypted format to backup my machine without Time Machine needing to encrypt the disk as a post-process operation. Is this possible?”
YES, this is possible, given what I said in “1.”, the best method that you can use (and that I use too) to achieve that is:
Format the disk in “Mac OS Extended (Journaled) Encrypted”
set the disk as the backup disk for TM, checking the option to set the backup as ENCRYPTED
Let it do the backup (which will not do additional encrypting processes etc.) and then you’re done!
Solution 2:
In your case for 2 TB drive it can take up to few days.
To keep it working overnight go to System Preferences > Energy Saver and leave the disk running.
Of course leave it plugged in.
Just to make sure it stays awake, you can use the caffeinate in Terminal.
You can check the status from the terminal with diskutil cs info /Volumes/<your drive name> and look for Conversion State and LV Conversion Progress