When I wake computer(iMac OS 10.5.8) from sleep it goes back to sleep again almost immediately. I then have to wake it again and it stays awake. This behavior just started. I notice at first awakening that there is activity on my external drive. The drive light then goes off and the computer goes back to sleep. The same thing happens with the external drive when I wake the computer the second time only it stays awake. Any thoughts?