CMD 649 could support IDE up to 133(Ultra DMA 6), but I don't know the capibility report for your Maxtor 40G and your IBM 40G. The OS will run the highest mode reported by your HDD.
CMD driver will degrade the mode if too many parity error occured until no any error issued again.
For your case, I suggest you check the event log to see if any error was generated. And check the operation mode of HDD from device management. I recommand you update the driver from Silicon Co.