Ok... I put in the other hard disk.
When i try and install morphos i get to the part where i choose automatic install or manual, i select the 20GB hard drive i installed and the computer freezes :O
What now?
You may find it easiest to:
1 - Remove your OSX hard drive;
2 - Install your 'new' hard drive (for MorphOS) - ensure to set it as 'master';
3 - Install MorphOS from the CD (use automatic partitioning to avoid problems) and then remove the CD from the drive;
4 - Reboot into MorphOS (to check it is working);
5 - Re-install your OSX hard-drive (either as 'Master' on a different PATA bus, or as 'Slave' on the same PATA bus;
Your Powermac should now boot to MorphOS by default, and you should now be able to choose to boot into OSX by holding 'Alt' during startup.
One thing to note is that automatic partitioning gives you a system drive which is only 511MB. You can always install your 3rd party applications to the 'Work' drive, or alternatively you can re-partition the drive AFTER INSTALLATION to suit your needs and copy your system files over to the new partition manually.
Hope that is helpful,
Rich
PS - If there are other faults causing crashes (as per your latest post) then all bets are off. I'd recommend that you figure out what the problem is under OSX (memtest or equivalent, try removing PCI expansion cards, try changing hard-drives, re-seating CPU module, cleaning away 5+ years-worth of dust, etc).
PPS - Good luck getting it sorted out, it really is worth it in the end. MorphOS is coming on leaps and bounds, and is my primary OS these days (I only keep Windows for my laptop).