Just solved a "hard disk not found" error not covered in the FAQ. After testing each drive separately, I found that the jumper I used for the second harddrive had wiggeled so loose, it didn't make a good connection anymore (it had been runnning fine for about 6 months). Lacking a proper, tight jumper, I soldered the two pins together. Worked like a charm.
So maybe add "check proper jumper connectivity" into the "hard disk not found" FAQ entry?