The same drives should work on any ST, and as far as I'm aware the original ST doesn't have anything different in terms of chips (although they are more likely to be socketed). If you can check the drives on another ST do so (check the cable as well) then do so. If they work then the problem may be with the drive port on the original ST, which may have some bad soldering (presuming the ST otherwise functions ok?). What you might also want to check is the drive ids as these may have been set to 1 (as opposed to 0) which would mean the ST would be seeing them as drive b rather than a, which may explain your problem.
Firebee, Falcon CT60, Milan 040, Falcon MkI, TT, Mega STe, Mega ST + Lots of STs of various flavours