Jack Mac,
I do this a little bit differently than you do but my setup is different too.
I use SuperDuper! to backup my boot disk to another disk of the same size. I don't want this disk mounted until the backup process starts and I wanted it unmounted when the backup process is finished.
First, I had to figure out how I was going to make sure the drive was unmounted until the backup took place. How does one do this easily? I decided I would use launchd to unmount the drive at start up.
I also decided to use a simple one line command to do this using "diskutil" and the backup disks UUID. The disk UUID does not change like the disk name can or the disk identifier does from boot to boot. Once I retrieved the UUID for my backup disk from Disk Utility, this simple command will unmount the backup disk:
/usr/sbin/diskutil eject 'EC408ECE-39F2-3A39-B628-E6575F2C422E'
I used
Lingon to make a "Users Daemon" to unmount the backup disk using that command.
First part of the problem solved.
SuperDuper! takes care of mounting the disk when the scheduled copy starts so that was easy.
The last issue was unmounting the disk when the backup was done. Again, SuperDuper! makes that easy too. I just have it run a script file with the command above in it.
That's all I needed.
For you, instead of using iCal and AppleScript, I would use Lingon to create a "Users Daemon" to mount the drive at a particular time each day just before the time you kick off the scheduled backup. Here is the mount command using the UUID:
/usr/sbin/diskutil mount 'EC408ECE-39F2-3A39-B628-E6575F2C422E'
Obviously the UUID would be different in your case.
S-