Why can’t I detect my old hard drive?

Okay. So in my computer, I originally owned a 40 GB IPE Seagate Barracuda. I ended up buying a hard drive from someone online, which is a SATA 500 GB Western Digital. After hooking the Western Digital to a SATA to IPE converter, I turned on the computer to get a ‘grub error’. I put in my Windows 7 Ultimate CD and formatted it. Now that it has downloaded all the updates and what not, it still does not detect the Seagate in the BIOS, Device Manager, or in My Computer whenever the Western Digital is hooked up. When the Western Digital is unplugged, I can resume using the Seagate freely. Also, why is my Western Digital making the computer run extremely slow? At times, the computer will freeze or shut down on its own.