- With the drive mounted, open Disk Utility and select the drive partition that will be the destination of the backup.
- Select Info, and make note of the Disk Identifier. In my case, it's disk1s10.
- Edit your crontab to contain the lines:
Replace the end of the first line, disk1s10 with the drive identifier from step one. Replace the end of the second line, '/Volumes/The Annex' with '/Volumes/name of your backup hard drive' (the quotes will handle any spaces in the drive's name).59 3 * * * /usr/sbin/diskutil mount /dev/disk1s10 15 4 * * * /usr/sbin/diskutil eject '/Volumes/The Annex'
Note that this won't be effective if you're backing up to a drive with more than one partition. Also, make sure that the unmounting time is somewhat beyond the time that your backups regularly finish. I've looked at the history of my backups and found that it always finishes in less than 10 minutes.

