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I just setup two backup drives for time machine in two different ways:

1) By clicking the "encrypted" checkbox in the time machine dialogue. After fininshing the backup, it spent a long time saying "encrypting..."

2) By formatting the drive as HFS+,Journalled,Encrypted using Disk Utility, and then keeping the the encrypted checkbox on in Time Machine (see note a).

The results are strange. The time machine preferences says that drive 2 is encrypted, however

diskutil cs list | egrep -i "size|status"


       Size (Total):       1999702540288 B (2.0 TB)
       Size (Converted):   -none-

The time machine preferences does not say that drive 1 is encrypted, though it too forever to do so. Perhaps that's because it's not mounted now.

(a) I also explored approach 2 while turning off the checkbox, thinking this would cause the material to be encrypted twice. However it said something indicating that it would decrypt the drive, so I stopped this.

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With option 1 you are encrypting the files, not the drive. Option 2 encrypts the whole drive.

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I don't think that's true. When I did option 1 on a new 1TB drive, the process took a very long time even though there were only a few MBs of files on it. When I ejected the drive, it no longer showed up when plugged into a Linux computer. The pastebin link shows what shows in "diskutils cs list" which looks pretty clear to me that it's encrypting the whole disk. – bobpaul Jan 25 '13 at 22:33

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