Troubleshooting an Imac G5 that won't mount on host computer in target mode.

I believe the internal harddrive on my Imac G5 has failed as the computer is stuck on a gray screen at startup and I am unable to repair the disk using disk utility on the OSX installation cd. I have the machine connected to another working mac with a firewire cable and am attempting to access the harddrive through target mode. The non-working mac starts in target mode but does not mount on the host computer. How can this be troubleshooted?