Concur....BIOS support is crucial...(laffing hysterically)..

Check the IDE cable too....sometimes, the master drive has to go on the
last coupling..and the slave, in between the motherboard and the master
drive...

It might go: on the IDE...MB to slave to master...in that order.

I had an Intel MB that did the same thing...and thats the way I had to
install the drives...

I dont think the hard drive capacity has that much to do with it...15
to 40....isnt that great a range...now if it was 15 to 120...might be
an issue...

Set the master to "master" on the jumpers..and slave to "slave" on the
varying drives...XP does the rest...

Hope it helps....