First of all, thank you so much Sam for posting this guide. It was super useful to me. My late 2015 iMac was running very slow and I decided to upgrade it with both a 1Tb SSD and a 128Gb flash drive (from a 2017 iMac). I followed your tutorial and everything is working well so far, after installing High Sierra in the flash unit.
There is only one thing that I’m a bit worried about. When running the Black magic test, read speeds are around 2500Mb/s while write speeds stay much lower, at 750Mb/s.
Is that normal?
I’ve also found an extra T10 in my late 2015 iMac