on most mb with sata on the older type the first hard drive that you want to boot from has to be on port 0. most sata scan from 0,1,2,3,4 then back to 0. on newer mb the bios will let you in the boot screen change what drive you want first. with the pre built system make sure both end of the sata data cable are not broken. sata data cables are small with a l shape alignment. the older ide drives have a 40 pin cable. if it an ide drive then there has to be a jumper for master/slave on some pre built they put the jumper on cable select. and where the drive is on the cable makes it master or slave. i would start by see if both drives show up in the bios if they do if your bios is new you should be able to set the older mec drive as first boot device.