Since I upgraded to 10.9, it takes a long time before my Mac Pro goes to sleep when I select the "sleep" command in the finder menu. Sometimes, when the computer awakes, it hangs and it is a hard crash, because the only solution is to press the power button. I have never experienced this with 10.6-10.8. My startup disk is an OWC SSD Mercury Extreme, I have never had problems with it, until now. 10.9.1 didn't solve the problem. Booting under 10.6.8 under another disk doesn't give this problem.