[dnanian]

Well -- the user needs to have a bit of discipline to get this to work: you really should have it set to back up at a specific time.

If the Mac is closed mid-copy, the same thing will happen as with any "in progress" file copy: it'll stop, and either work on resume (if the external is connected) or fail (if not). It's not something that should generally be done.

The way to do a Smart Update is covered in some detail in the User's Guide, so take a quick browse over the first few sections... I think you'll find it useful.