"First you have a spare SATA port in this model so you don't need to remove your current HD or the optical drive.
So you can follow this IFIXIT guide: Installing iMac Intel 21.5"" EMC 2428 Dual Hard Drive Kit. Go to Step 46 as you can see you are wedging the second drive (HD or SSD) behind the optical drive. Once you've put the SSD in you can then create a Fusion drive across both your HD and SSD gaining the benefits of each.
If you decide to just replace your HD for a SSD thats fine to. But you'll need to use this special cable: OWC In-line Digital Thermal Sensor for iMac 2011 Hard Drive Upgrade.
Frankly, I think the best option here is swapping out your HD for a SSHD (hybrid HD). Here you'll gain the performance of the SSD but the storage capacity of a HD.
Here's the Seagate web site: Seagate Desktop SSHD and here is the spec sheet for the SSHD series: Desktop SSHD specsheet."
"Given that this thread dates from the 2014/2017 timeframe, I think the SSHD recommendation no longer applies.
I put in a Seagate SSHD back then and have been thoroughly unimpressed with its performance from day one. Prices of SSDs have plummeted since then which makes SSHDs no longer viable."
Hi, i have same issue, Beware iMac 27 mid 2011, don't work with samsung 860 EVO (Because samsung use unsupported new controller) also same situation can be with 850 EVO (i didn't check with this series), so optimal for upgrade without any risk will be use 840 EVO series. Symptomatic is next: When you connect 860 EVO (I check it) or 850 EVO (I didn't check it 850 EVO, so info for 850 EVO maybe not correct). You can start from internet recovery but you cannot do fusion drive, for fusion drive you should start from USB recovery, yes you can do fusion drive with 860 EVO or 850 EVO but then when will be recovery from time machine you will se error and time machine is stop, then if you will try install new system to you new fusion formatted disk, you will cannot install, not sierra, not sierra high, maybe el captain will work but you will see many error when install it (6 hours maybe), if you still install some OS don't start to be happy, because will be many errors and white screen. So by the way. If you try to instal os to pure ssd, it's also will be with error. So conclusion don't use 860 EVO and 850 EVO with iMac 27 mid 2011 (I have core i5 3,1). So your SSD will work with USB 2.0 on your iMac, or need to buy thunderbolt ssd adapter or thunderbolt usb 3.0
"If you install a 3rd party SSD you will need to download software to enable the TRIM support. I have been using this program for the last couple of years Trim Enabler.
Otherwise totally go with the recommendations in the previous answer. Personally I am more partial to SSD's especially since the prices have only been falling. I highly recommend the Samsung 840 or 850 Pro drives.
"I installed a 1 TB hybrid drive in my 2009 27"" iMac, the reason I did this was so I could remove the sensor off the old 1 TB drive and put it on the new hybrid drive (SSHD). That way the temp sensor does not run the fans at super full speed.
Also at the time I read about SSD's they need to be purged once in a while, maybe not now but 3 years ago I think that was the case. So with the hybrid drive as it needs to it can purge the caching flash part of the drive.
In the end the programs and files I use all the time are right there and the stuff I don't use all the time is still in the deeper storage on the HD side, so it starts up quick and programs I use all the time open right up.
This seems to work well for me. I have a 2 TB drive on the time machine that I only turn on once in a while."
"@Dan - I've got a question about some of the information you've posted. Background: I have a mid-2011 21.5"" iMac. In Disk Utility, the hard drive (factory-original Seagate ST3500418AS, 500GB) reports S.M.A.R.T. status as ""failing."" In researching how to properly replace the HD, I've seen all the discussion (here and elsewhere) about the proprietary temperature sensing firmware etc. that was added in the 2011 iMacs.
In your comments above, you mention a couple times that replacing the factory HDD with a SSHD would not require the OWC in-line sensor cable (""get a SSHD that has the builtin sensor and gives you similar performance of a SSD"" -- 1/6/2015; ""the SSHD has the needed built-in thermal sensor (you won't need the OWC cable)"" -- 1/6/2015). But then elsewhere, you say that even with an SSHD, you'll still need the OWC in-line sensor cable (""Don't forget you'll need the OWC in-line sensor if you go with the SSHD!"" -- 9/16/2017; ""You still need the OWC in-line thermal sensor cable."" -- 9/16/2017).
I'm not trying to call you out at all...I'm just confused--which is it? Could you clarify for those of us that might be in this situation? Are there any SSHD's that do have a built-in sensor and appropriate firmware, such that no OWC cable is required? If so, do you know which brands/models are suitable? I am trying to avoid buying the OWC cable if possible, given the somewhat steep price for such a small part ($40+). I would much rather spend that extra money on a larger/faster disk drive that already has a built-in thermal sensor.
