[dnanian]

Well, it's unclear what drive you were running from. If you were able to eject the drive at all, then you were running from some other drive. And if you were running on a schedule, and had a backup set to run from the internal to the external, it would fail if you were running from the external.

Given that, the only 'automatic' way the copy could have run would be from the internal to the external (at least unless you changed things)... and since you didn't indicate you received any errors, that would mean you weren't running from the external.

Did you try to 'fix' this yourself by restoring from the external?