It depends on how the other devices are configured..
You have a choice whether to install the new HD on the primary or the secondary IDE channel. Primary channel typically contains the OS hd and your CDROM is on the secondary channel.
If you put your new HD on the same cable as your existing one then you have to set the OS HD as master and the new one as slave. Also, make sure the master is connectd to the black plug and the slave to the grey one (that is if you are using the new 80 wire IDE cable).
If you decide to use the other channel, set the CDROM as slave and the music HD as master.
Usually the jumpers are labelled as MA, SL and CS. If you can't figure out how to set master and slave on those, you'd better not mess with the internals of your computer. If your drives have more complicated jumpers or there are no labels, check the HD manufaturer's website for jumper settings.
