Just tried to install it but didnt work. I got to where you select the HDD, and it is not displayed in there. Needless to say with OSX I did not have any of these problems.
You could try couple of things:
1. Check that the devices are correctly jumpered for master/slave. MorphOS might be pickier about this.
2. You could try different 80-pin ATA cable. Mac OS X might be able to downgrade the DMA modes in case of flaky cable.
3. You could try the HDD connected to a different ATA connector. Many Power Mac models have multiple ATA controllers.
Also we'd be interested to hear the exact model of the system, full showconfig and pciscan verbose output.