10.5: Run Time Machine on a single drive: “
Contrary to popular belief, Time Machine does not require a second physical drive (or network drive) to store its backup files. It seems to be quite happy backing up to a secondary partition on the primary hard drive.
Use Disk Utility to create a dedicated HFS+ partition for Time Machine that is as large or larger than the partition(s) you want to back up (Disk Utility in 10.5 allows live resizing of the active system partition – yay!). After the partition has been created, use the Time Machine preferences pane to select it as the backup destination.
When you select the backup partition, you will receive a warning about how backing up to the same physical disk is not a safe as backing up to a second physical drive, but you are allowed to click through and do it anyway.
As warned, using a secondary partition as your backup does not provide the same hardware redu…

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(Via Mac OS X Hints.)