Mac Freezes when left IDLE
[Perhaps this should be a comment rather than an answer, but I don’t have quite enough reputation points yet to comment. Sigh.]

I wonder whether the log is showing cause or effect here. It looks like you have an external drive attached. Since the problem happens at the one-hour mark, and since there are drive-related errors immediately pre-freeze, perhaps the problem is caused by the external drive responding incorrectly when being put to sleep by the system.

Could you try bringing up the “Energy Saver” System Preference panel and unchecking “Put hard disks to sleep when possible” — does the iMac still freeze?