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 x'''2 Lanes''' interface)
* Mid-2015 - PCIe 2.0 x'''4 Lanes''' interface)
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It sounds like you have a smaller SSD in the newer system. At this time there are no SSD upgrade possible by third parties
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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: [http://www.samsung.com/global/business/semiconductor/minisite/SSD/global/html/portablessdt3/overview.html|Samsung Portable SSD T3]. We bought a few and are finding them quite nice (USB3).
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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-synchronization/id414421761?mt=12}File Synchronization]
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 x'''2 Lanes''' interface)
* Mid-2015 - PCIe 2.0 x'''4 Lanes''' interface)