Harddrives should be on the same cable, since they are faster than DVD/CD-RWs (normally). The transfer rate wouldn't be faster than the slowest item. This would be the DVD/CD-RW (usually ATA33, unless they have become faster recently), while the HDs are probably ATA100 or ATA133. So you would get a significant performance drop by putting HDs and DVD/CD-RWs on the same cable...

Stig