I have a Gateway GM5478, about 3 years old, and today got a blue screen error (dont think I have ever had that error on this computer before.) After trying to reboot in normal mode I still was unable to boot in Safe Mode with the process completely stalling. Then after power down it wouldnt recognize the hard drive any longer.
I took HD to Comp Repair place and they verified they couldn't access it at all. So I went to Best Buy bought a new WD 1TB Caviar BLack. Got it home, STILL isnt recognized in BIOS.
I replaced STAT cable. Rechecked all wires including power cable. Every 1 out of 10 times the BIOS WILL detect drive but very rarely. And of course when it does not detect the HD the drive does not spin at all.
So, it isnt the drive. It seems to me it has to be the motherboard which has the built in SATA controllers or the BIOS? Sometimes during initial part of boot process it does not even find a controller. Other times it will display that Sata 0,1,2 there is no device.
I would try clearing the CMOS and resetting the BIOS to defaults after that. If still not working, likely the mobo is dying on you. Replacing the mobo with a compatible one (not necessarily identical one) should solve the problem, if you are willing to go that route. If not, a new PC is the way to go...
Best bet is to try another PSU (borrow one from a friend or another comp); just to ensure that isn't the culprit.
I assume if you run up with just 1 stick of RAM and the CPU connected it all goes as expected?
Also, if you're trying it in SATA 0, try it in SATA 1 or 2. Also check the jumper on the back of the new (supposedly working) drive, and ensure its set to 3GB/s, not 6, as your motherboard may not support that speed, and therefore can't recognise it in BIOS.