If you read through these forums (forgive me for such a foolish suggestion) you'll find hundreds of strange things that might keep a hunk of media from booting. Your theory that your system might only be willing to boot from the master device seemed as good as any to me. But since you've now convinced yourself that can not be it because of something a completely different system does, why not try an experiment that would be even less work than moving the two little jumpers: try booting a Knoppix CD in the DVD drive. If it boots then it seems that the BIOS is able to boot from a slave device and so the master/slave thing likely (but not certainly) isn't the issue. If your BIOS will not boot a CD either from the slave device then that's very likely the issue. You did say previously:
Quote Originally Posted by Chthonic Miasma
3.7-3.9 all boot on this computer fine, and 3.8.2 only had a hardware issue, but the dvd doesnt even give a try
but you didn't tell us which drive you were booting the CD from. So you might have already done this but we have no way of knowing since you didn't say which drive you used to boot the CD. Your "but the dvd doesnt even give a try" statement sure seems to indicate that the BIOS isn't even spinning up the slave drive.