In my case I never used Time Machine, it is always turned off since it was introduced to OS X. It is for sure not Time Machine problem, at least not in my case.
I may be wrong, but based on other posts in this forumm and Apple Discussions forums the problem affects mostly large capacity WD Green drives with IntelliPower. Maybe the culprit is IntelliPower - reports of drives not going to sleep may support this. Just my 2 cents…
