Sorry, I know there are loads of threads on sleep problems, but I can't find any that relate specifically to my problem. I have an early 2009 iMac with a Western Digital MyBook Home hard drive connected via FireWire, using Time Machine. I can put my iMac to sleep manually and it will stay asleep, but it won't go to sleep after the set time (1 hour). I have discovered that the MyBook is the problem (if I switch it off, the iMac will sleep after 1 hour). Can anyone help? Many thanks in advance.