Obratio sam se WD i odgovor je slijedeći:
If your EIDE controller supports asynchronous data transfer rates (different rates for each device on the controller), you can add a second device that uses a different ATA rate without affecting the rate of an existing device. If the controller does not support asynchronous data transfers, the system will run at the fastest rate supported by both devices. For example, if you added an ATA/66 drive to a system with an existing ATA/100 drive, both drives would operate at ATA/66. Check with your controller card manufacturer to determine if your card supports asynchronous transfers. This may also be referred to as "independent device timing".
Znači ovisi o MB i ne postoji generalno pravilo.