I decided to start using a second TM backup disk. I opted for the disk to be encrypted. The backup was reasonably quick, given that it was a full backup of about 250Gbytes -but then it said 'encrypting...' and has only got to 46% after about 7 hours!!
Is this normal? I guess it is disk access intensive, but 7 hours...!
Those sorts of times are usually associated with encrypting a disk that already contains a lot of data (which may or may not be TM backups). The trick is to encrypt the disk while it's empty, the additional time encrypting data written/read is negligible at the time of writing/reading (there's substantial hardware support for encryption).
The mystery is why your disk is being encrypted after the backup. Where did you ask to encrypt the disk?
With a newly formatted disk, I opened TM preferences, clicked 'add or remove a backup disk'. The next window allows you to select the disk for TM and in the bottom LH corner there is a tick box for 'encrypt backups'.
So I assumed that it would encrypt the disk then do the TM backup -but no!
BTW, the encryption phase lasted over 12 hours (I went to bed at 85%). A clue perhaps is that when the machine was awake the light on the disk flashed rapidly, but when asleep it flashed slowly. I tried various things* in the energy saver window of system preferences, but none seemed to stop the mac sleeping, even though it was always on mains power.
*untick 'put HDs to sleep when possible. 'untick prevent computer from sleeping...' tried both settings of 'power nap...'
What is actually happening seems clear (!): the first backup was written unencrypted and was then encrypted afterwards. Very strange.
I have always formatted my backup drives as encrypted before using them. Doing that was partly because I didn't entirely understand what TM would do for 'encrypt back-ups' and I did want the entire disk encrypted. I now claim inspiration and recommend that approach as easier to understand and lacking this strange behaviour.
Can anyone else shed any light on TM's apparently odd behaviour?
Richard I 