Chosen Solution
I saw that the 1 TB Fusion Drive SSD is connected via NVM Express and the 2 TB Fusion Drive Connects via PCI Express. Isn’t the NVM Express the faster connection?
This can be a bit confusing! First lets review what a Fusion Drive is: The word ‘fuse’ give us a clue here, we are linking (fusing) two discrete drives within the OS so they are seen as one. In this case a SATA III (6.0 Gb/s) traditional spinning hard drive and either a PCIe 2.0 x 4 lane NVM-Express blade SSD (in the current model) or in the previous model a PCIe 2 lane blade SSD. iMac 27" 3.3 GHz i5 (5K, Mid-2015) iMac 27" 3.2 GHz i5 (5K, Late-2015) For a comparison here is a benchmark between them: 2014/Mid 2015 iMac 5K: writes at ~716 MB/sec and reads at ~766 MB/sec. Late 2015 iMac 5K: writes at ~1520 MB/sec and reads at ~2080 MB/sec. While the benchmark shows the faster 4 lane PCIe NVM interfaced SSD is faster than the older 2 lane PCIe. The width of the connection has a lot to do with it (2 vs 4 lanes). To be clear here you can’t use the newer SSD in the older system the system does not offer a 4 lane PCIe interface.
