|All modern IDE systems support "independent device timing" which means that if two drives that operate at different speeds are connected to the same cable/channel, they will run at their respective speeds. In other words, if you have an ATA100 HDD & an ATA33 ATAPI drive connected to the same channel, the HDD will run at ATA100 speed while the ATAPI drive will run at ATA33 speed. Years ago, before independent device timing became standard, this was a no-no because the faster drive would be forced to run at the speed of the slower drive. Put it this way:|
With Independent Device Timing:
ATA100 + ATA33 = ATA100 + ATA33
Without Independent Device Timing:
ATA100 + ATA33 = ATA33 + ATA33
However, whenever possibile, it's generally best to put drives on separate channels to attain best data transfer rates. This is because the transfer rates are better between channels (primary to secondary) than they are between 2 devices sharing the same channel.
"By its very nature, each IDE/ATA channel can only deal with one request, to one device, at a time. You cannot even begin a second request, even to a different drive, until the first request is completed. This means that if you put two devices on the same channel, they must share it. In practical terms, this means that any time one device is in use, the other must remain silent. In contrast, two disks on two different IDE/ATA channels can process requests simultaneously on most motherboards. The bottom line is that the best way to configure multiple devices is to make each of them a single drive on its own channel, if this is possible."
See the following:
Independent Master/Slave Device Timing
Performance Factors and Tradeoffs in Configuring for Multiple Devices