Upgrading HDD to SSD...is the Fusion SSD still used afterward?

Thanks Dan! Alas in my case leaving the 3TB HDD alone isn't an option as it is returning SMART errors. I'm fully backed up and will replace it this coming week.

The question is with what? I'm looking at this kit from iFixit with a 2TB SSD (that also includes the temp sensor and cable). I know the improvement in performance will be thrilling. The downside is that I lose 33% of the storage of my 3TB HDD. And while 4TB SSD are available, the price difference is huge.

Muddying the water is that I could go with a massive 3.5" HDD for what the 2TB SSD would cost. Of course the performance gain of the SSD is certainly the long-term correct answer. But again that trade off of losing so much storage on day one. That's what I get for maxing out my BTO late-2013 iMac 27 back in the day. For most users a 2TB SSD would be a gain in storage and performance. Oh well!

Question- Regarding my original 128gb blade SSD...will it even mount after the SSD conversion?

@kupe - So what are you looking for? Speed or deep storage? Or do you want both!

What I do for many is install a bigger blade SSD for the boot drive, this will have your OS, apps and depending on your workflow enough free space for OS/App caching and scratch space. It's not your deep storage! This is what I would go with Custom Apple blade SSD mSATA - 512GB or Custom Apple blade SSD mSATA - 768GB. Then for your 3.5" HDD I would jump to a Seagate FireCuda ST8000DX001 8TB 7200 RPM 256MB Cache SATA 6.0Gb/s for the deep storage. You still need the thermal sensor.

Take that old 128GB and fling it out the window as its useless! It's too small for anyone serious and likely well worn by now.

Thanks again @danj

Interesting- if I go with the Seagate Firecuda HDD you say I still need the thermal sensor? Is that because it doesn't have the temp sensor that the original Apple/Seagate HDD has? Now, I know from the iFixit tutorial where the temp sensor attaches to an SDD...any tips on where to attach it to the Firecuda HDD?

Also- just FYI the Apple blade SSDs you linked to show Not Compatible with Late 2013 iMac 27 iMac 14,2 EMC 2639.

@kupe - Sorry I grabbed the 2012 model here's the 2013 models 2012 27" iMac blade SSD's

And yes you need the sensor which plugs into the same connection on the FireCuda HDD and in the special Apple Seagate HDD) Review the Step-34 in this guide Sostituzione hard drive iMac Intel 27" EMC 2639 Don't over think this! The 2.5" and the 3.5" drives use the exact same SATA connection!

@danj I really appreciate all your help! I'm still leaning toward going with an SSD to replace the HDD in my iMac 27. I've reviewed that guide a dozen times and read every comment. What I'm hung up on is the issue of splitting/breaking the Fusion Drive. Surprising that it doesn't come up in either the Guide or in any of the comments. My gut sense is that nobody is doing it. Maybe it's done already when you swap in a new drive?

And speaking of SSD, I'm seeing 4TB units that are at least somewhat affordable. Do you have any thoughts on this WD 4TB SSD?