Why might a PC see only some bootable DOS disks as bootable?

[plushbug]

I have a 32-bit box here, that I built back in 2005, and recently converted to what I'd call a "shell" PC--ie., no fixed HDD inside, whatever HDD I'm running in it just sits in a removable tray.  This seems to work well with all the drives I've ever built previously, I've been able to drop in and run everything from DOS 6.22 through Win 3.1, Win 95, Win 98, RH Linux, Win XP Pro, but here's the problem:  messing around with an un-built drive I'd like to rebuild, I've discovered that when I try to boot my 2005 hardware from bootable DOS diskettes, I can't get anything earlier than my Win 95 boot disk to be recognized as bootable.  Everything from my original DOS 6.22 setup disks on down, until I get to that Win 95 boot disk either I get the "non-system disk" error, or if I happen to have the XP drive in the tray, it seems to ignore that there's anything in the floppy drive at all--floppy is marked as first boot device in the BIOS, but the system just buzzes ahead and runs XP.  I'm finding this seriously puzzling, given the disks I've been trying are all ones I've used and had work fine with builds in pre-2005 boxes now long dismantled and gone (except for their HDDs).

Any thoughts as to what I'm missing, or being blindingly ignorant about, here?

I only EVER learn by doing, screwing up, redoing...

[BC_Programmer]

perhaps you should double-check that the floppy disks are bootable- with an older machine, for example.

[plushbug]

At this point, don't have an older box handy to do this with. The 1997-vintage Pentium I used to use all these disks with regularly (except for the "real" DOS 6.22 boot disk I created last weekend, following the instructions here on CH) got recycled pretty much down to its hard drives back in 2005, so I'll have to scrounge for anything sufficiently vintage to make a good test.  Best I can say right now is that disks tried have worked reliably on previous outings, and been stored securely away from magnets and lightening strikes since last use.

Off to scrounge...

[BC_Programmer]

Another option, if possible- try a DOS boot disk from bootdisk.com

It could be the fact that a lot of Install diskettes used a format called "DMF" which allowed for 1.72MB of data on a 1.44" floppy. Unfortunately they were extremely unstable as far as holding data, and especially for booting.

So what do do if the bootdisk.com image boots?

Well- if your lucky, the DOS setup disk simply couldn't boot, and with luck you can read the disk, and thus run setup.

you could also try chkdsk- make sure the write protect tab is ON, as well.