Mac OS X Server 10.5: About using drives for network Time Machine backups
With Mac OS X Server version 10.5, you can store network Time Machine backups on both internal and external hard drives. If you choose an external USB or FireWire drive for your backups, take care to prevent accidental disconnection during the backup process. Before physically disconnecting the drive, be sure eject the drive icon in the Finder. If the drive does not eject, wait until the backup is complete and try again.