If a drive does not get up to its normal speed, the BIOS may treat it as a failure and not log it in. This can be unexpectedly caused by:
1. Erratic power connectors.
2. Worn bearings or dirt in the drive HDD motor.
3. A weak power supply
And the are the usual every day things, which you shooed have already checked out..
1. Somebody three the PC off the desktop.
2. At last night's party they put beer in the PC fan.
3. Your ferrets got in and chewed wires.