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I recently upgraded my hard drive from the stock Hitachi 250GB to a Western Digital 320GB. I restored from Time Machine.

I plugged in my hard drive today and left my hard drive to back up. Surprisingly, when I came back, the backups were being deleted.

Why?

It was the same installation. Now my unrecoverable backups are gone.

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Were you (close to) out of space on the external drive? –  Nathan Greenstein Apr 2 '11 at 17:13
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The signature likely changed with the new drive, but my experience is that it wouldn't back up rather than deleting them. Perhaps it is different if using Time Machine without a Time Capsule? Not having to deal with multiple machines, maybe it just overwrites what is there if the signature is different? Just a guess. –  g . Apr 2 '11 at 21:40
    
Nathan - 90GB left on the HD. –  JFW Apr 3 '11 at 8:11

1 Answer 1

up vote 3 down vote accepted

After a full restore, a full backup is preformed (always). It's by design. This is also documented on the wonderful Podini - About Time Machine site, section TM is doing a full backup for no good reason. Other circumstances, that may trigger full backup, are listed too.

I quote:

And Time Machine may do a full backup after any of the following:
* A full restore, whether from Time Machine or a "clone" (probably)

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