I have an old Dell Inspiron which runs fairly well except that two of the internal hard drives become inaccessible after a while.
At startup both drives run fine until the computer is left idle. When I return to the computer the drives are no longer visible in the drive list. However they are still detected under Device Manager but are listed as not being initialized. After rebooting both drives are accessible again. I have also left the computer idle but with one of the drives transferring data only to come back and find the transfer failed and the drive inaccessible.
One of the problematic drives is a 320GB WD SATA connected directly to the motherboard. The other is an 80GB WD(?) PATA running off a Promise PCI controller card along with another identical drive on the same IDE cable which does not experience any problems.