Upgrading the internal PCIe SSD in a 2015 21.5" iMac Fusion Drive

This was a challenge. My Fusion Drive performance was dropping as it was getting quite long in the tooth. So I resolved to replace the internal PCIe on the logic board with a larger one than the 30Gb unit that had been part of the Fusion Drive setup; and no longer use the Fusion Drive but run entirely from the internal PCIe SSD.

After much taking apart and putting together, I established that the iMac doesn't like using an NVME SSD with adapter (the setup that works perfectly in a MacBook Pro 2013 to 2015); it would not even start up, in fact it would not even display *any* of the diagnostic LEDs—a fact which surprised me and made me look elsewhere at the first for the cause of the problem.

However when I fitted an original Apple 512Gb SSD gleaned from a MacBook Pro 2015, everything worked perfectly and I now have a throughput of over 1500 Mb/sec versus upgrading the SATA drive (lots easier of course) which would have given about 500 Mb/sec.

Just a heads-up in case anyone else thinks of upgrading the PCIe SSD in a 21.5" Fusion Drive iMac 2015.

I rather suspect that the same might hold true for the 27", happy to be corrected on that issue

Guy (happily using Macs for 36 years)