You have to boot from OS 9. Pro Tools will not work in OS X or Classic mode within OS X: You have a couple of options you could try: Choose OS 9.2.2 on the hard drive that came with your new computer as the startup disk, restart in OS 9, and install Pro Tools on that drive. Or you may be able to update the OS on your previous drive to 9.2.2, which I'm sure is required for the latest G4 to boot. If your previous drive does not have Mac OS 9.2.2 on it that may be why you can't select it as the boot drive. No guarantees that either will work though, as the new G4's have not yet been qualified: See this DUC topic as well: