I tried to replace an iBook Dual Usb G3 cdrom with a dvd drive but hit a few problems.
The ibook cdrom is configured as a slave and the hard disk is master.
The dvd I had was a toshiba that was master only and couldn't be changed to be a slave so I experimented a bit and found out if the hard disk is changed to be slave then the slave hard disk dvd master combination works.
The hard disk (mine was a toshiba mk2018gas) needs a jumper to make it act as a slave.
Take apart ibook pdf at this site
toshiba mk2018gas hard disk jumper pdf