The PC in question is a P4, custom built with an Intel D945PVS motherboard.
I built it back in 2006 and its hard drive died once, and I replaced it with another hard drive of the same type. (A maxtor 250GB SATA)
I wanted to upgrade to an SSD, so I unplugged the old hard drive, plugged in the SSD, and booted my usb stick (with unetbootin) and installed Debian.
Installation completed, but on reboot, PC reports that no bootable media could be found.
Drive can be seen in BIOS. BIOS drive is set to ACHI, although changing it to IDE or RAID did not help.
I bought a 500GB mechanical drive (seagate) just for kicks, and threw it in. Same Debian install, still won't boot int it.
I know the installation media is not at fault as I've used it to install Debian to other PCs in the past, both before and after I tried it on this particular PC.
I plugged the old drive back in, and it spins up and loads Ubuntu (the OS that was on it before) perfectly.
What could be wrong?