they sound like they are IDE interface
one must be master M jumper
one must be slave S jumper
however sometimes there is a mismatch and some ide interfaces need the two drives to co-operate a bit, there may be a bios setting to enable/disable 32 bit transfers this may affect operation one way or the other otherwise you need the help of the CS jumper cable select
sometimes the secondary must be set to CS jumper cable select as it wont otherwise co-operate
if the two drives do not work set as M and CS then swop the two drives about, in the jumper sense
ie make the other one M and the other other one CS
quite often you find that only one of the two drives will work as the real master and that
would explain what you are experiencing.
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note some drives use UDMA 33 and some UDMA 66 and some UDMA 100
this is another possible mismatch drives should drop their speed to accomodate the lowest so thats really why you may need to switch which one is in control