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I'm making an encrypted Time Machine backup for the first time. I had assumed it would encrypt the data as it was writing it, but now I see that (after leaving it alone for several hours) the Preferences pane for Time Machine shows a progress bar with the word "Encrypting…". Does that mean it writes the backup in cleartext and then encrypts it all in a separate phase?

I'm using 10.9.5.

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I assume you're backing up to a directly attached drive (USB drive or something similar)? When you select a local drive for encrypted backups, it converts the entire drive (or at least the volume you selected) to encrypted format, then starts backing up your data into it.

Basically, it's encrypting the current contents (probably most blank, but it encrypts blank space as well). Once that one-time conversion is finished, it'll start backing your data up, encrypting that as it goes.

  • Oh, I see. Yes, it's a local USB drive and is totally blank except for the partitioning I did to it. I thought when I started the backup it showed something other than "Encrypting…" under the progress bar, which led me to believe that the encryption part was at the end; but what you say makes more sense. I probably misunderstood the beginning part of it. – echristopherson Sep 20 '15 at 21:24
  • Hmm, no, I was wrong -- when I actually view the backup volume, it has ten timestamped backup directories, starting this morning and increasing by about an hour each time. And when I look in them, they do have my content, and it doesn't appear to be encrypted (although I guess if the encryption is whole-volume it wouldn't be evident once I've successfully mounted the volume). Is it possible the system has already encrypted the used data on this volume, but is continuing to encrypt just the free space in the background? – echristopherson Sep 20 '15 at 21:29
  • @echristopherson: That may be; my impression was that it waited for encryption to finish before starting backups, but they may've changed that since I tried it. Also, you're correct that the the encryption isn't visible in the finder -- while the volume is mounted (which requires the password), data is automatically decrypted as it's read from the disk (e.g. as you look at files in the Finder) and encrypted as it's written (e.g. as backup snapshots are written, you manually copy files to the disk, etc). – Gordon Davisson Sep 20 '15 at 23:12

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