Your new harddisk probably needs an 80 pin IDE cable to function properly. Are you using one of those?

Also with an 80pin cable, it's necessary to put the master device on the end of the cable and the slave device in the middle. This wasn't so the case with 40 pin cable.

If that doesn't work you can try switching devices, but that probably won't work.

Overall, I've found that it's generally a bad idea (now especially with the 80 pin cables) to mix harddisks and non-harddisk IDE devices (cd-rom etc.) on the same IDE channel (cable).

Your best bet is probably to buy one of those ATA/133 PCI cards that adds an entirely new IDE channel to your PC. You know, like the ones Promise makes. They aren't too expensive and function well.