(The one thing that I don't like about Scanner is that it basically blocks until it's done scanning, with no apparent way to cancel, but it does give a progress indicator, and you at least know when it's done.)

This wasn't the experience I had. I was able to select a different drive icon while it was still scanning, and it seemed to Do The Right Thing: it stopped the existing scan and began scanning the drive I clicked on. Maybe I downloaded a newer version than the one you're running?