I am trying to fix a HP Pavillion 521N computer. It was working fine until one day the computer said that there was no boot device. Fine, I removed the hard drive and put it on USB controller and ran check disk on another computer. It found several problems and fixed them. The drive is accessible on another computer. I put it back in the HP and the bios doesn't recognize the HDD. The drive jumper is set to Cable Select. Fine, I replaced the Hard Drive with a known working drive. Bios still doesn't recognize a hard drive. I switched the jumper to Master, Slave and back to CS. No Joy. There is a CD-RW drive in the Secondary Master position and a DVD drive on the Secondary Slave position. Only the DVD drive in the Slave position is being recognized. I tried various hard drives in various jumper positions, with various IDE cables. I put a Lite-On DVD-RW on the Primary Master position and it is recognized, but not the original HP CD-RW, which is recognized on another computer.. I then updated the bios, successful but the first bios screen shows the same bios #. I tried running the HDD on the Secondary cable as Master and as a Secondary Slave. No Joy I am stumped, in my 10 years of computer repair, I haven't seen this one. Any other ideas??