I TRIED TO BOOT MY CPU W/ a WD 160HD AS A SECOND DRIVE. THE MAIN DRIVE JUMPER WAS SET TO CABLE SELECT AND I DIDN'T NOTICE BUT THE 160 WAS SET TO DUAL MASTER. WHEN I TRIED TO BOOT I GOT THREE LONG BEEPS AND THAT'S ALL IT DOES NOW EVEN WHEN I PUT THE ORIGINAL DRIVES BACK AS THEY WERE (IT WON'T BOOT UP). DOES MIXING THE JUMPER SETTINGS CAUSE A CATASTROPHIC FAILURE OR CAN I GET IT BACK TO WHERE IT WAS PREVIOUSLY?no, it may have changed your bios setting an not sure what one to boot too.What brand is the motherboard.......remove and reseat memory, cards ....you may have moved them when working on the HARD drivescan you tell me how to set that if it won't boot up?ALSO, I did remove and reseat the memory as well as the vieo cards....that was in the list of possible causes that I found along the way.....to no avail.....This the motherboard INFO. The computer is an older Gateway.....late 2001 actually.

Intel (Monterrey) motherboard - 1.8-GHz [Part #2514224] Here's a step by step

http://www.pcmech.com/article/installing-a-hard-drive-step-by-step/