I think I finally fixed the problem by installing the apmd package. :-\ The problem was, I am guessing, that the hard drive went to sleep at some random time (despite it being explicitly told not to in the BIOS) and regardless of the disk activity (so even my 15 min dd cronjob did not fix it) and nothing was there to detect it and tell the system that the drive was not dead, but just sleeping. I'm not sure what apmd does when the drive goes to sleep, but it seems to do something useful.
Anyway, not a kernel bug (unless you think it should have managed without apmd,) so please close this and sorry to have wasted your time. George.