[dnanian]

SuperDuper! does not interact with the drive at a hardware level in ANY way. The warning that we give when you 'stop' is simply to let you know that the drive will be in an "unknown" state: that is, you can't know what files are there and what aren't, or if you're in the middle of creating an image, the intermediate products won't be deleted. It has nothing to do with it being "unstable" at all.

It looks to me like you had a hardware failure of some sort, which would also explain why things were going so slowly.