I have a 2009 iMac, installed an SSD in place of the stock hard drive. OS and apps on the SSD. All 'files' & media is stored on the stock hard drive which is now installed in an external USB enclosure. This is where my problems start. I get this message all the time. Within System Preferences I have deselected 'put hard disks to sleep', but it still happens. Any other ideas? What's really annoying is that every time this happens my iMac wakes from sleep and will not automatically return to sleep. I often wake up and is wide awake.