HDD: Single, Master, Primary, end of cable
CD-R/W: Master, Secondary, end of cable
CD-ROM: Slave, Secondary, middle connector
There you have it 🙂
This relieves the bottleneck on the most bandwidth hungry device by giving it a channel all it's own (The HDD)
It also provides a full-bandwidth flow of data from the HDD to the CD-R/W provided the CD-ROM is not in use.
As long as you aren't copying at 40x from CD-to-CD it should be perfect (CD-to-CD is NEVER the way to go. If anything, use Daemon-Tools for a *perfect* "virtual" CD drive compatible with almost every image type and copyprotection, then copy from that drive.