I posted this a couple of weeks ago:
A strategy for managing SD! jobs via AppleScript
There's a tension in any similar question to this: It is very appealing to say "use this line", so most posts on any forum are of such a form, but then one ends up with a solution that for various reasons works only 90% of the time. A 50 line solution such as my post stands a far better chance of working all of the time, with all the gotchas ironed out, but such solutions generally scare most people off.
There are several reasons to want to unmount backup volumes. The delays in spinning them back up for every "open" or "save" command from an application are very annoying. Sure, this is terrible OS code, the OS should cache enough information about spun down drives to implement "lazy" access, only spinning them up if the user explicitly visits one. But, hey, the OS isn't written that way.
Another reason is security: An unmounted drive can't get hosed as easily.
