problem is, there's no perfect way to do it. the important thing to keep in mind is that only one device on a channel can be active at any given time. so, if you're doing something in photoshop, which benefits from a scratch disk, well, it won't really help if the scratch disk is on the same ide channel because you can't access both hdd's anyway. hence the axiom each hdd should have it's own channel.
do they have to be master-well, this is a complicated question. there is no priority, so to speak, of a master device over a slave in terms of controlling the bus, so they are badly named in this respect. master and slave can make a difference to the os, drive letter assignments, etc.
but, if you are only rarely using the 2nd hdd as backup only, and you watch alot of dvds, you could argue to make the dvd the secondary master and put both hdds on the primary ide.
i kind of depends on how you're going to use your hdds, cdr/dvd, and system
sohcrates is right though-do a search and you'll find numerous posts on the subject.
