NEC 4550A and UDMA problem - weird

Looking for an insight here, friends. This is what happened: had a IDE Hard drive (master) and this NEC dvd drive (slave) on the primary channel; there is another DVD drive (LG ) as master on the Secondary IDE channel - all working fine.
T h e n . . I removed the IDE HDD, and fitted a much speedier PATA2 hdd to the mo/bo; made the NEC DVD drive the master on the IDE primary. The effect was to slow the DVD drive right down to about half - or less - normal speed - every operation done in jerks! The other DVD drive was quite unaffected, and the computer works fine otherwise!!
Have tried all sorts to find the fault - changing cables, different dvd drive nothing helps. The only difference that I can find is that in the BIOS, the Primary IDE has UDMA 4 enabled (greyed out) while the sec IDE ch only has UDMA 2 enabled (also greyed out). Now I know that UDMA 4 is twice the speed that UDMA 2 is! Is this the cause of my fault? If so, how do I change the greyed-out UDMA value, anyone, please?? If not the prob, then what might be? Help guys!! Brian

Ooops - so sorry - that PATA should read SATA. I’m a fool! Brian