time machine turn off encryption
How do I turn off encryption for a Time Machine backup?
I have an external drive that I have been using unencrypted for a year or so. I recently connected it to a new macbook air in order to use as a Time Machine disk for that laptop, in addition to all of the other stuff it has stored on it (e.g. photos).
When setting up Time Machine, I opted to make the backup encrypted. Everything works fine. However, for other reasons, I would now like to turn encryption *off* (the reason is that encrypting the Time Machine back-ups is encrypting the entire disk, and that's inconvenient for other reasons).
I do not see a "turn off encryption" option with Time Machine -- i.e. no analog to the setting I initially chose to enable. If I open Disk Utility, the File menu contains an option to "Turn Off Encryption" but that option is grayed out -- I cannot select it.
One possible explanation I thought of is that Time Machine was not done encrypting the disk, and that perhaps you can't unencrypt a volume until it is done encrypting. However, I think encryption is complete (it no longer states that it is busy encrypting when I connect the disk to my laptop, but I also don't see a status indicator that encryption is complete either).
And so I repeat: how do I turn off encryption?
MacBook Air, OS X Mountain Lion (10.8.2)