The only really foolproof solution, I think, is to keep backups of knoppix-data.img, for instance, on a USB harddisk. Should something go wrong, one may always go back to a working version. I think that reservation trick is a smart safety measure, but lots of things may go wrong. As for sanity checks, what kinds of checks are run on the fs in knoppix-data.img? And what about fragmentation etc - does that have any practical significance there?