Chosen Solution
I just replaced the 128 GB Apple blade SSD in my late 2014 5k 27" iMac with a 500 GBg Apple blade SSD (SM0512L) breaking the original Fusion Drive set. I also replaced the 1TB HDD with a Crucial SSD. The Crucial SSD via the HD SATA port is connected at full speed (6.0 Gb/s), very happy. But the new Apple SSD shows a Link Width x2 (@5GTs). The SSD is a 2018 model and PCIe x4 - why am I only getting x2 (and hence lower speeds)? Is it because my iMac doesn’t support PCIe x4 ? The new SSD also now shows under NVMe. I’ve spent the past few days (and literally hours) reading through various forum postings, but can’t find anything about my particular iMac model and the ‘support’ for Apple SSD’s. The speed for the new SSD is much better than the original - 730 for both read/write, compared to 290 write, 580 read on the older model, but I was expecting double those figures - probably stupidly. Anybody know what the late 2014 27" iMac actually supports?
I think your issue maybe the systems firmware level and the MacOS version you are running. I don’t have this system so I can’t be sure, but I would get to Sierra if you are not there now and then give High Sierra a try. Both are updated when you do a OS upgrade. WARNING! High Sierra will automatically switch your boot SSD drive to APFS (it may also alter your second SSD drive (haven’t tested that). So keep that in mind! Update (04/17/2018) Here’s a useful diagram:
As you can see there are two paths for ACHI services the legacy SATA (Red path) and AHCI (Orange path). I don’t think Apple uses 4 lanes in ACHI mode. It would seem to be a waste as SATA to block translation services on the SSD (AHCI controller) can only go so fast, and technically it can’t exceed 6 Gb/s.
