|There were/are reasons why certain devices shouldn't share the same channel, but for the most part, that relates to older hardware.|
Here's a few quotes from PCGuide.com:
"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."
That statement should help explain why a burner should be on the opposite channel of the drive it's copying from. If you copy CDs "on the fly" (from reader to writer), the reader should be on the primary channel & the writer (burner) should be on the 2ndary channel. That way there's a constant data flow. If both devices share the same channel, there will be constant buffering, so not only will it take longer, but the buffering increases the risks of producing "coasters". The same thing applies to transferring data from the HDD to the burner.
IDE contollers have used "Independent Device Timing" since the days of the socket 7. In other words, the controller supports running the master and slave device on the same channel at different transfer rates. An ATA33 device will NOT cause an ATA100 device to slow to ATA33 speed...they will operate independently of each other.
"Since the transfer modes associated with the IDE/ATA interface are constantly being improved, new devices support faster transfer modes than older ones do. In addition, hard disks often support faster transfer modes than ATAPI devices such as optical drives do. Yet, these devices can be combined on the same IDE/ATA channel, raising the question of compatibility when the devices are together.
The ability of an IDE/ATA channel to operate a master and slave device using different transfer modes is called independent device timing. The hard disk controllers integrated on modern chipsets all pretty much support independent timing, as do modern add-in controllers, but this was not always the case."