The most recent discussion on this topic I can find here is from last May.
I (based partly on Adam’s success replacing Apple’s tiny 28 or 32 GB “blade” SSD in the processor direct slot) did the same and repurposed the rotating platter drive in the drive bay to a place to store Time Machine backups (a fool’s errand, of course, if the iMac fails because of some problem that makes it unable to boot).
Currently I’m involved in a discussion on Mac-L with another list member there who is considering doing the same thing, and that discussion has gone into the weeds as regards the logical structure of Apple’s “Fusion Drives.” Apple seems always to have advertised the total storage as being equivalent to the size of the rotating platter hard disk, but I’ve been told on Mac-L that the two storage elements are logically additive. I’d assumed that the tiny blade SSD in the processor direct slot actually worked like a programmatically customized cache, storing at least the boot elements of the OS and data that’s used a LOT. However, given the decreasing expense of SSD 'drives" that would fit in the drive bay, perhaps using solid state memory in BOTH locations, e.g., OWC’s 1 TB stick in the processor direct slot and a 2.5" “drive-sized” 1 TB internal SSD in the drive bay and mating them into a 2 TB “Fusion Drive” would create a VERY nice upgrade, if indeed 1 + 1 = 2 rather than 1 + 1 = 1, but in 2 different places
Of course, if the SSD in the processor direct slot functions as a cache rather than additive storage, it might actually REDUCE overall performance.
So, my bottom-line question: does a 1 TB NVMe stick in the processor direct slot + a 1 TB SATA drive in the drive bay, if fused together, create a 1 or 2 TB storage device? Has anyone here done it?
And, for people contemplating such an undertaking remember that, after ripping off the display, then exenterating the entire machine, before you start reassembly, the only things left in the case will be the BT and WiFi antenna modules and the screws holding the monitor base to the case. it’s tedious, but when you’re done you can do your best imitation of Tom Hanks in Cast Away when he erupts in self-congratulatory joy (OK, he did have an audience of 1 soccer ball), beating his chest and proclaiming “I HAVE MADE FIRE!!!”
On the other hand, after you’ve done this once, successfully, you’ll be tempted to go back in for other tweaks or for other reasons. In my case, because I started with an 8 GB RAM machine, I also boosted RAM to 32 GB, and 18 months later I began getting kernel panics, which REMBER blamed on faulty memory. OWC replaced that quickly and at no cost to me, but I’d gotten careless and overconfident, so when putting everything back together I munged two of the MOST bulletproof reconnections (the fairly robust connectors at the ends of the WiFi and BT antenna cables that pin on to the Broadcom BT/WiFi board), and once I reapplied fresh blue painters tape to the bezels of the case (a nice upgrade for those who want to boast their spelunking skills) I discovered I no longer had Bluetooth! (Well, actually, I had very primitive Bluetooth—my keyboard sort of worked, but none of my Apple BT mice would pair, my BT headphones would try to pair but just make a few horrible noises and then give up).
Knowing what I’d done wrong, I looked into replacing the Antenna cables and discovered that Apple’s authorized repair recommendation was a replacement of the entire metal case, because the antennas themselves are fused to the periphery of the case. Fortunately someone else who’s done this stuff told me that with the assistance of a a pair of illuminated magnifying spectacles I might be able to wedge a tiny jeweler’s screwdriver into the cable-end connectors, which basically work like a tiny gold plated alligator clamp to grasp the pins on the WiFi/BT board and bend those tiny blades back into position, and (miracle of miracles) that WORKED!
(Chapter Next): all my Googling and Duck Duck Going on this topic, realizing with a mixture of dread and elation, facing the prospect of telling my wife that I just MIGHT “need” to retire my iMac and purchase an M1x Apple Silicon iMac as soon as they appear next spring, led me to the realization that the processor in my machine is SOCKETED, not soldered to the motherboard, and that some have actually upgraded the Quad Core i5 to a Core i7 processor. Those discussions feature lots of worry about whether the cooling would be adequate, whether the fans would be spinning raucously non-stop, etc., and my guess is that I won’t go there, but the MAIN reason for that is that the processors themselves are still VERY expensive, and the performance gains seem to be VERY modest.