I replaced my original HD in my Imac 21" mid 2010 and experienced the fan problem (full speed) as well. Instead of going into a new disassembly and error and trial exercise to quiet down the fans, I got me a license on the HDD fan control. That worked very well and now my Mac is not noisy any more. I rely on the program statement that temperature now is controlled through the S.M.A.R.T technology for sufficient protection of my IMac.
16/12/2015
Roald