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I’ve purchased a Mac Mini 2014 with 256 Gb PCIe SSD, Can i add a 2.5 inch SATA SSD to this case with an iFixit drive tray and SATA cable?

Yes, you can add a second HDD or SDD (SATA) to a SSD PCIe blade only system. You’ll need these parts: Mac mini Dual Drive Kit and this cable: Mac mini A1347 (Mid 2011-Late 2012) Lower SATA Cable The tray is setup to accept the drive.