SSD to Replace original HDD in 27" iMac mid 2011 (12,2) model?

I recently replaced my dead internal HD with an SSD. I had been running my 2013 iMac via an external SSD. I sourced all the tools, SSD, temp sensor kit, adhesive kit, Adapt-a-tray and tools from CanadaRam. They are a reseller for OWC parts. I then followed the OWC videos to replace the drive. After replacing the drive, I copied from my external SSD to the newly installed internal. The entire process was wonderfully painless.

Can you clarify further --

"I have already installed a previous 500GB SSD (Samsung 850EVO 500GB) in the iMac (using the OWC video) a few yrs. ago."

-- did you indicate you have already installed 850EVO in this iMac? Where did you install it? The optical drive spot? or somewhere else?

I would suggest NOT to spend much money on 2011 machines though...

However, from my previous doodling experience -- I much prefer in-line senor adapter for 2011 iMac. Just plug in and forget about it. The in-line sensor adapter is needed for the HDD's SATA connection. That is extra cost for this DIY project.

Software hack (fan control) is another method (no cost) -- you have to ask yourself: do you want to spend the extra? or software route, or divert it all the cost and invest in newer system?

You only need the temperature sensor adapter when replacing and installing the SSD into the main drive bay of the iMac. If you are replacing the optical drive with an SSD, then use the OWC adapter for mounting & connecting an SSD into the optical drive bay. You will have everything you need either way if you follow OWC's instructions and guidelines.

OWC are the experts on iMac upgrades. They have take the time to research and develop the necessary extra parts to make the iMac run properly with a non-Apple drive installed. I'm not aware of any other vendor who has more thoroughly investigated and tested the Mac upgrade options they sell. Plus OWC provides excellent tech support.

If your hard drive is failing, then contact a professional data recovery service such as Drive Savers or Ontrack to see if they can recover any data if you don't have a backup. Both vendors provide free estimates and both are recommended by Apple.

Normal utilities (even data recovery apps) are not able to recover data from a failing hard drive because macOS and those apps are unable to handle all of the errors produced by a failing hard drive. The more you attempt to access a failing hard drive (or even leave a failing hard drive powered on) the worse the drive failure becomes where even a professional data recovery service will be unable to recover any data from the drive.

You usually only get one chance to recover data from a failing hard drive so make sure to choose wisely.

You should always have frequent and regular backups.

You are welcome.