Sorry No the SSD's are not interchangeable. The speed difference is due to the number of lanes the newer system has:
- Mid-2014 - PCIe 2.0 x2 Lanes interface)
- Mid-2015 - PCIe 2.0 x4 Lanes interface)
It sounds like you have a smaller SSD in the newer system. At this time there are no SSD upgrade possible by third parties
There are a few different ways you could move things around here. Have you looked at getting an external drive? How about going with an external SSD drive like this one: Samsung Portable SSD T3. We bought a few and are finding them quite nice (USB3).
If space is not the issue then how about using a sync app to keep the data files mirrored between the systems. Here's one we use: https://itunes.apple.com/us/app/file-syn...
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