This issue continues to a terribly misunderstood one.
There has never really been issues pairing fast UDMA mode devices with slower mode UDMA devices, except in extreme cases. They always work, except for extremely troublesome devices, which would likely have issues with ANY device paired with it.
The problem (so often talked about) occurs when you are using both the slower device and the fast device simulataneously for a realtime/time critical purpose and the fast device has to wait for a slow device to finish it’s transfer and to release the bus. The bus effectively (but not actually) drops the speed to lowest speed (at the same time operating the faster device at the higher speed) due to the delays incurred by the device transmitting at the slower speed.
This was of particular concern before burners started incorporating burnproof into them … but is still an issue, despite burnproof, because some of the solutions rely on the burning program turning the devices burnproof on … rather than wholely and intelligently implementing the burnproof method solely within the device.
So of course, it’s still not recommended to pair a UDMA2 device with a UDMA5 device, each device will still work fine seperately, and likely ok together also, especially if they are only optical devices with maximum 25MB/s transfer rates, but at 16x, it’s likely slow downs will occur, because it is close to the maximum bandwidth (when switching between the two).
It’s still not recommended to pair a UDMA2 optical drive with a UDMA5/6 HD, because these two devices can easily saturate the available bandwidth, and will be forever waiting for the other to finish with the bus.
Aka -> 24/32MB/s (UDMA2) = 75% use -> leaves 25% use for UDMA5/100 = 25MB/s.
Woohoo … just works … for DVD optical drives only.
In reality, both drives will be forced to slow down, due to delays caused by waiting for the other device to release the bus.
HD’s can easily saturate the IDE bus to the point that the UDMA2 device will be forced to slowdown though.
Luckily, DVD optical devices only reach 22-24MB/s for a very short period of time, and not likely to be simultaneously, so it’ll probably work ok regardless.