About two weeks ago I replaced the hard drive for my G4 desktop. Long story short, it's already messed up. Upon startup, the screen stays on the white screen w/grey apple displaying the progress indicator...and it stays on this screen w/o booting. Now, I don't have an OS disk, BUT I have a copy that I downloaded that is on an external hard drive. I tried holding option at startup, but the external hard drive doesn't show. Is there anyway to startup my computer using this copy on an external hard drive rather than purchasing another copy of OSX? Thanks in advance.