The other 'nasty' is machines that will only take a limited number of hard disk types (geometries). Sometimes only 2 or 3 drives are supported, and you can bet that they'll either be almost impossible to find a drive with all parameters in excess of the ones needed, or it will actually verify that the drive has the specified geometry (e.g. checking that a '6 head' drive doesn't work when heads >5 are selected).

Getting the right spare drive for one of those can really drive you insane.