So, I recently got a very nice vintage ATX mid-tower case, and moved a Pentium III 500 motherboard out of the one I had it in (a rather badly yellowed mid-tower with only 3 5 1/4" open drive bays, the one external 3.5" bay was specially shaped for a 3.5" fdd; I had planned a case upgrade for months).

However, I recently noticed a strange issue with the motherboard in particular (an old Gateway Tabor III motherboard). It seems that the BIOS will detect the CD drive, however, it will not boot off of any CDs, even with the boot order changed to place the CD drive as the first boot option. I have also noticed that an old version of 'smart boot manager'? won't detect the CD drive at all, and neither will Windows NT 4.0 setup. I actually did notice this issue originally, but because I was installing Windows 98 SE, I was able to boot off of a Windows 95 boot disk and copy the setup files from the CD to the HDD through DOS. 98 SE managed to used the CD drive just fine.

That's what's odd about this. DOS can see the CD drive, install the device drivers for it, and copy files from it. However, the BIOS refuses to boot from the CD drive, and Windows NT 4.0 setup started from floppy disks will not detect the CD drive, even though it has done so on the same motherboard.

I've tried the following steps to troubleshoot this:
- Hard Reset the BIOS by unplugging it and removing the CMOS battery for ~20 seconds before turning it on without the CMOS battery and then re-inserting it after turning it back off. (I know there is probably a CMOS reset jumper on there; I just haven't looked for it yet).

- Change out the CD drive itself (the original that was in there was a DVD/CD drive, I changed it out for a standard CD-ROM drive).

- Change out the IDE cable with a known good one.

- I got desperate enough to try and run MemTest 86+ to see if it was caused by bad RAM, even though I knew it was unlikely.

I will also state that there was possibly a traumatic event for this motherboard, wherein I put a Pentium III Coppermine 600mhz to upgrade it: it originally came with a Pentium III Katmai 500mhz (the voltages are different: Katmai uses 2V and Coppermine uses 1.6V). However, I find it unlikely that that did anything to either the motherboard itself or the processor, as the BIOS even recognized the 600mhz PIII and I used it with that processor just fine for quite a while.

I'm out of ideas for this, honestly. The computer has been decommissioned, sitting on its side for a while as I've been trying to figure this out. I don't think it's a bad IDE port because, as I said earlier, it copies files back and forth just fine under DOS/Win98, and the BIOS recognizes the CD drive.