Help solve mystery of: iMac Fusion drive SSD slot

[studioj20]

Yes wow, it is $$$. $900 for the 2tb and it probably doesn’t fit anyway.
It does seem using the terminal command at the link above to disable some sleep functions does in fact make the Aura Pro X2 upgrade compatible. I left it on over night and no kernel panics.

 

[mdgm]

There's a long thread about failed EFI firmware updates on the 2015 iMacs: iMac 17,1 (Retina 5K, 27-inch, Late 2015) EFI firmware update failures

I do wonder whether that's related. If you can get the EFI firmware updated to the latest then one would hope the NVMe drive would work well.