However, when transferring data from one drive to another, the slowest drive will limit the transfer rate.
Given the same spindle speed and other general similarities, an ATA66 and an ATA100 will run at the same speed anyway. A common misconception is that a UDMA100/133 drive is faster than UDMA66. Not true. The UDMA rating is for the interface
, not the drive itself. Very few, if any, UDMA100/133 drives will ever exceed 66 MB/Sec transfer speed. More like 45-55.
In any case, you can mix and match as you like on any current MB.