Mac OS X 10.5: Time Machine - "The backup volume could not be found" alert
If you erase your designated Time Machine backup disk, or if your external hard drive has a sleep function that does not wake for Time Machine backups, Time Machine may alert you that "The backup volume could not be found" and backups may not be made.
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For hard drives that are erased
From the Apple menu, choose System Preferences.
From the View menu, choose Time Machine.
Click "Change Disk...".
Select your back up location again.
For hard drives that sleep and do not wake for Time Machine backups