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I have 2 x 1 TB drive that I use interchangeably. 1 holds Video and the other Photos. I only have 1 connected when I'm working (using Premiere or Lightroom). I have a 2 TB NAS for making redundant copy of the other 2 drives.So far I've been doing the copying manually.

I'm considering of using Time Machine to make proper backup sets (though I'm always only interested in the latest set). I know that I can set Time Machine to backup to NAS over the network and include any external drives connected to the mac at the time.

QUestion now is:

  1. Will Time Machine "craps out" if it can't find the external drive?
  2. Does Time Machine handle backing up the external drives - one at a time - only when it's connected - without thinking the files/volume had been deleted?
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I've deleted my answer as I misinterpreted your first questions as asking what would happen if the drive you were backing up /to/ was not available. – Nick Hawes Sep 30 '12 at 9:44
  1. I also have my external HDD connected from time to time. To answer to Afmrak, Time Machine only prepares (stashes) backups once it finds out you connected your Time Machine HDD. So if you haven't connected your HDD for some time, it will take a while for Time Machine to prepare and make backup.

  2. I have never been backing up external volumes.

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up vote 1 down vote accepted

Finally tried doing it myself:

  1. Time Machine handles non connected drives gracefully. No error msg popping out. When the drive is connected then it would start the backing up process.
  2. Time Machine seems to handle partitions/volumes independent to one another. So if only either one of the harddrives are connected, it will happily process the one that's connected
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To answer your first question:

I'm pretty sure Mac OS X TimeMachine continues to run when the external drive isn't present by stashing file diffs on the local drive. When the external TimeMachine drive is reconnected it pushes the stashed changes over.

I can't vouch for the storage saving schemes (if any) it has while backing up locally, but the feature is there.

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