I have an old 1992 Pentium Pro 180 Mhz computer that I use for all my backup storage. It has 2 120 Gb hard disks and each of my other computers run backups to these hard disks twice a week. On Thursday morning, I noticed that it had failed to be available for some backups. The 2 backup hard disks had failed to be recognized by the boot up process. I unplugged the disk drive cables and and reseated the cables and re-booted. The 2 drives were again recognized by the boot process, but then the boot up hard disk failed to boot up.
Finally, last night, distraught about the possible demise of my old ’92, I unplugged all the cables and noticed that I hadn’t plugged all of them all the way in.
Success. Back-ups are back up.