Storage Dimensions:2.5" (9.5 mm)/Proprietary*Storage Interface:Serial ATA (6 Gb/s)/PCIe*
Details:*This model has a Serial ATA III (6 Gb/s) connector for a 2.5" hard drive or SSD as well as a proprietary PCIe connector for a "blade" SSD, although as noted by OWC, the cable to connect a "blade" SSD to the PCIe connector is not present unless the system is configured with a "Fusion Drive" at the time of initial purchase.
Here’s the blade upgrade:
Mac mini Late 2014 SSD Replacement
Here’s the hard drive/SSD upgrade:
Mac mini Late 2014 Hard Drive Replacement
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
2