The SSD on my iMac crashed a few weeks ago now.  The iMac originally came with a 256gb SSD and a 1TB internal HD for storage as well.  This post is NOT about “adding” an SSD to a machine without one.  This post IS about replacing your SSD if it goes bad…  There are ton’s of tutorials out there about adding one but I couldn’t find a single one about replacing a bad one.  So, first off, let’s cover what you’ll need.  I used a few torx bits.  T10 is used the most.  There are 8 T10 screws holding the screen in and 4 holding in the DVD drive.   Once you get the DVD out the old SSD is actually sitting right behind it.  You’ll need an T8.5 to swap the rails from the old drive onto the new drive.  You’ll need and SSD drive.  I used the closest direct replacement by going with a sandisk 240gb pro SSD.  Here’s the link.  I did not use the suction cups everyone mentions using to get the glass/plexi off the front of the computer.  I just got my fingernails under there and carefully pulled it forward.  It’s held on by magnets so it comes off w/o much drama.  I would follow the advice of OWC in the video below and use your box to hold the glass and screen once it’s out, it’s AMAZINGLY HELPFUL!!!  Once the 8 screws are our of the screen itself leans forward and then you have to remove 4 wires/cables.  Go slow and easy with their connections.  Once that’s done the DVD is obvious (there’s once sensor wire to remove then it’s data and power).  Once that’s out the old SSD is right behind it with 3 screws holding it in.  Remove it from the case and remove it’s wires.  Plug in the new drive and reverse the order…

I highly recommend having another computer available with this video playing:

you’re going to do what they do through the 4:50 mark but then you’re done.  Also, if your drive dies on you so you can’t boot the OS, I installed the OS on the TB HD just so I could get at the data in the SSD.  Some copied fine some things didn’t.  But you can also see the files in folders as well so at least you’ll know what you’re missing…  You DO NOT NEED any additional cables, the SSD I linked above uses the cables the iMac had from the factory.  Once you’ve got in installed go ahead and turn it back on and install your OS on it.

Here’s another great tutorial but with pictures -vs- the video above: https://www.ifixit.com/Guide/Installing+iMac+Intel+27-Inch+EMC+2429+Dual+Hard+Drive+Kit/7575

follow it to step 16 but once that DVD drive is out just pull the old SSD out from behind it.

This of course has made me improve my backup game…  Everything important is on other drives included a RAIDed setup for my images.  But two things I lost in this that I’m now backing up…  First is my current LR catalog.  I generally keep it on the SSD so it runs faster.  Never thought to back it up.  Now it’s only april and that only has my ratings and basic edits really so it’s not a huge loss but it’s worth mentioning (and adding to a time-capsule backup to the RAID).  The other is my LR/PS actions and droplets…  They’re gone and a PITA to remake.  Not impossible but more work than I wanted to have to do!  I guess a third would be the entire SSD or at least the OS if you want so when this happens you can just copy it back onto the new drive instead of having to start from scratch…  But I don’t mind a fresh install to be honest, it lets me get rid of a bunch of programs etc that I wasn’t using and only put back what I really need/use.

Good Luck!