The iMac's hard disk drive has a temperature sensor that must be used. Even worse, the drive's firmware is specifically altered by Apple with proprietary code so generic drives will exhibit the exact symptoms you are describing.
Obviously there is a way to fix this but not necessarily with the drive you bought. Now that you know what the problem is, do some research (here is a place to start: http://www.tuaw.com/2011/05/12/owc-replacing-main-hard-drive-with-third-party-is -not-an-option/) and you'll find a solution.
If I were to replace a drive I would go with an Apple vendor who knows about this idiosyncrasy. OWC would be my first choice.
Edit: OWC explains this further: http://blog.macsales.com/10146-apple-further-restricts-upgrade-options-on-new-im acs
Message was edited by: John Galt
I took all the precautions and asked all the questions,
Then I trust you saw this before you did the replacement. Apparently, it's not just a question of getting a drive from the same manufacturer to use with the proprietary cable.
It's acting as if the temp sensor is not connected. I have it bookmarked amidst thousands of bookmarks but can't remember the name right now. There is some program out there that, in the absence of getting data from the internal sensor, will regulate the fan speed based on the SMART temp. If I think of it, I'll post back.
Here it is. It's meant for the 2011 iMac Apple drives, but I don't see why it wouldn't work in your situation.
BTW, I think John Galt is thinking of the 2011 iMac drives, not the late 2009s, which are not quite as impossible.
Perhaps it's SMC Fan Control you're thinking of?
I don't have it, I don't know how it determines the temperature, and I don't know how it works. It seems using it would be like treating the symptom rather than fixing the problem, but that's up to Frag to determine.