Hi, I have read that it's possibe to add a second hard drive to the iMac mid 2011. There is supposed to be an extra 2,5"...

One thing though, theres also freewheels that won't come off with the gear sprockets =there's no need to take the freewheel off in that case. You should check this page: http://sheldonbrown.com/gloss_ca-g.html#cassette