Mac will not sleep unless external monitor unplugged

Updated Mid 2011, 27" iMac to Yosemite. Computer would not sleep. Checked network settings, sleep settings, unplugged external drives. Finally slept when I unplugged an external monitor. I'm using an old cinema display with an adapter into the Thunderbolt port. Plugged it back in, Mac wouldn't sleep.
Is there a fix for this?

Apparently there is a task running that wants periodic disk access. Have you tried simply looking at Activity Monitor to see if there's a task running that might have been forgotten?
Try creating a temporary, "test" user account. Log out of yours and into the test account. Does that fix anything?
There are lots of things that could cause your Cube's insomnia. I know you've already looked for everything you can think of, but here are a couple of links that may help:
http://discussions.apple.com/thread.jspa?messageID=11238080
(Don't pay attention to the fact it's in the Snow Leopard forum, the causes are all the same. Also, I don't recommend arbitrarily clearing cache files as one user suggested - I can't imagine that's your problem anyway.)
Mac OS X: Why your Mac might not sleep or stay in sleep mode
Good luck!

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