I think I may be getting somewhere with the problems I have been having with my 20 inch Alu iMac. Occasionally, It will shut down completely (as in turning off), and sometimes it will refuse to wake from sleep. Often after these occurrences, pushing the start button to reboot the system will cause the machine to 'whir up' but will never get as far as the 'chime'. I have now realised that if my external hard drives are UNPLUGGED I have none of these issues. I have a 250Gb Western Digital MyBook and a 500Gb Seagate Any further thoughts Alu iMac owners?