I have an Imac flat panel Power PC G4, OS 10.1.5. I loaded extra memory to give me 1024mb. I wanted to load a higher OS. The Apple people told me that I could load Tiger, but when I try, the computer locks up at the 'Select Destination' window. The only thing highlighted at that window is 'Go Back'. The HD icon is not highlighted, nor are the 'continue' or 'options' buttons. At the bottom of the window, it says " Some volumes are unavailable until installer has finished processing" Is there a way to work around this?