Ah, the venerable disk defragmenter! I ran it once, and it was slowly progressing. 97%, 98%, 99%, almost done now I thought! 100%, 101%, 102%, uh oh, that can't be good... but maybe it was just a rounding error in the calculation of the percentage? I stopped it at 110%, but the damage was already done, all the sectors shuffled so much the filesystem was effectively unreadable.