hi blithe.spirits,

i suggest you test each hard drive and corresponding IDE cable on a different machine. use Ultimatebootcd to run the hard drive diagnostics for western digital.

if the hard drive and ide pair do not show any errors on ultimatebootcd, then see if a different power supply unit would remove the error.

im saying this because it maybe possible that the power supply unit has intermittent electrical problems that may have affected the hard drive -- hence the random cases when the hard drive is not detected in bios.

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i had experienced a similar case before. i was able to test that the hard drive, ide cable, and power supply were all good. however, it was the ide circuit on the motherboard that went bad.