Because if you have the Burner and the CD player on the same channel, you will not get optimum speeds out of your burner when you do CD to CD copies.
For CD to CD copies its not a issue, and it never has been as long as both the CD drives are in one of the various DMA modes. (PIO modes suck too much CPU time) You'll have no issues doing CD to CD copies with both drives on the same channel, even at 52x (7,200KB/sec) which is the speed of the fastest burners today... Try it yourself and you'll see
Copying between 2 hardrives on the same channel is not that big of a issue ethier. Test it for yourself by copying files with both drives on the same channel and then again with both drives on different channels. With both drives on the same chanell you'll see maybe a 1 second difference, if even that.
While its quite true only one device on a channel can send or receive data at a time, each device will only transfer a few kilobytes of data before releasing the channel for the other device to use. Devices "take turns" using the channel....
if you hook a cdrw or cd with the HD it will bring down the transfer rate to the lowest speed which in this case is the cdrw or cd.
This was only true with first generation IDE controllers. This was when IDE supported only hardrives. Most all IDE controllers have supported independent timing since about the time ATAPI support (protocol non-hardrive devices use) was introduced.
clicknext - Just arrange your IDE devices in whatever manner is most convienient for you. Don't worry about performance "issues" everyone goes on about. As long as all your hardrives and CD-RW/CD-ROMs are in one of the few DMA modes you'll have no problems.