Hi all. I have a mid-2011 21" iMac i5 (2.5 GHz) and did all my research before replacing my hard drive. I went with a 240 GB OWC Mercury SSD and watched all the videos from iFixit and OWC. My install seems to be in the minority of what other users did. I put mine in place of the original HD (as apposed to in place of the optical drive or in addition to the HD and optical drives) I did this because I really don't like spinning drives (noise, heat, slowness and eventual failure) and 240 is plenty of working space for me. Anyway, I got the drive in place (in an Icy Dock shell) and reattached the LCD cables and put the glass screen back on. Now on bootup there is a loud fan noise in my iMac. So loud I can't stand it. I did download SSDFanControl and it keeps the HD and optical drive fans at a nice low speed. But the noise was still there! I brought up another app called iStat Pro and it showed that my CPU fan is revving @ 3900 rpm. What causes this and what can I do about it? I read the very lengthy thread here about SSD installs and not many mention the CPU fan. Please help if you have a solution! Thank you.