Sorry, I have just checked and the 27" takes two hard drives (unlike the 21.5 which only takes one), so you can ignore most of my above post.
The 2011 models used integrated temperature sensors in the hard drive that did not use a separate cable, so installing a non-Apple drive (including SSDs) will require an OWC Hard Drive sensor kit.
Congratulations! I had this exact model for the last 4 years. Other than the Seagate HHD recall ( i.e. Affected some units....Apple replaced the HHD under this recall), the iMac had no issues. I just recently sold it (for $900) after removing the additional SSD and replaced with a late 2015 27 inch version. So, you got a good deal!
I see some of your other posts below regarding adding SSD. Here is what I did. I left the HHD in its place, but installed an additional SSD via the instructions in this video from OWC ( i.e. And using a kit purchased from OWC that included everything needed....around $35 at the time). You then have lots of options on how you want to use the HHD/SSD combo. You can create your own fusion drive, install OSX and apps on the SSD and "manually" manage what data you keep on the SSD versus what data you keep on the SSD, etc..... There are plenty of smart people on this forum that can help you with the steps involved (and pros/cons) of the various options on how to best us the HHD/SSD combo. So, my recommendation is to keep the HHD installed and just install an additional SSD (i.e. Then you have both to work with).
The whole process was fairly simple if you follow the instructions in the video carefully. You have to be careful (i.e. Lots of fragile things to disconnect/reconnect), but very simple really.
Video:
http://eshop.macsales.com/installvideos/imac_mid27_2010_ssddiy/
Yes, it's possible to keep both drives (i.e. I did...as per the video in my link) and it's possible to "pick' the new SSD as the drive that the computer boots from "with much faster "boot up" and other application "startup" speed. Other's on this forum, much more knowledable that I about the "mechanics" of installing a fresh copy of OS X on the SSD as well as installing the Apps on the SSD can hopefully chime in for specifics...as well as pros/cons of the various options of using a combined hdd/ssd configuration.interesting, so if I understand your post, it's possible to keep both drives and pick the new SSD as the drive that the computer boots from without any delay in speed?
Yes, it's possible to keep both drives (i.e. I did...as per the video in my link) and it's possible to "pick' the new SSD as the drive that the computer boots from "with much faster "boot up" and other application "startup" speed. Other's on this forum, much more knowledable that I about the "mechanics" of installing a fresh copy of OS X on the SSD as well as installing the Apps on the SSD can hopefully chime in for specifics...as well as pros/cons of the various options of using a combined hdd/ssd configuration.
Just as the previous poster said: Time Machine BU can be set to back up both.....only limited by the size of the drive used for your BUHow does it work with time machine backups then? Do you only backup the drive that the OS is installed on?