Hi, its a temp sensor apple puts in their hard drives thats not present in other drives not from apple. There is software designed to deal with this problem, its not free but works well. The website also explains the problem,
The software is iMac HDD Fan Control, the website is www.hddfancontrol.com.
Oh and there is a free demo to see if it works for you.
EDIT:
Sorry I did not realize the age of the question.
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were you trying to run diagnostics with the D key?
crwdns2934271:0crwdnd2934271:0 mayer crwdne2934271:0