On my old secondary computer my setup was

Motherboard: Chaintech VNF2-350
Master 0: HDS722516VLAT80 with Windows XP SP3
Slave 0: WD800BB-53CCB0

But then I started getting Machine Check Exceptions and my motherboard ultimately perished. So I replaced that with a Foxconn K8S755A-6ELRS. But this new motherboard won't detect the hard drive.

It will hang at the part that says "Detecting IDE drives", sometimes it will give up and display "0" where the drive is supposed to be, other times it will hang until you unplug the cable.

If it's on the same channel as the Hitachi drive with XP on it, the BIOS recognizes the other drive, but fails to boot from it.

If they are on different channels, it boots the XP drive but will see no hint of the drive on the other channel., even in Disk Management or Device Manager.

I also tried connecting it to my other computer's Intel DP35DP mobo, and it was able to detect/read/write and all that, though it does have a few bad sectors.

Any help?

"though it does have a few bad sectors"...

ANY hard drive with ANY bad sectors should be replaced ASAP. If you are trying to boot from a hard drive loaded with an operating system from another computer, don't! Select the drive you want as a boot drive, format it and install an OS of your choice fresh

I'm trying to make it last as long as I can until hard drive prices go down (or I just replace it all with a laptop or something)

You list 2 hard drives, do both of them have issues? You didn't answer my question. Are you trying to boot a hard drive with Windows already installed from another PC?

Only one has the issue. The other one with XP came from the same computer, and I can boot from it, if that drive is in by itself.

Okay the drive with XP is bad or it needs a low-level format. If you can use the second drive as a boot drive, install XP on that one fresh