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Is the cable all is needed to add the PCIe capability to a current Mac Mini 2014? Or do i need the whole hdd enclosure? Assuming, i can just use some tap to hold the PCIe cable and M.2 in place

It depends on which drive option you bought. A HDD model only will just need the cable Solid State Drive Flex Cable - P/N 076-00018 and here’s the IFIXIT guide to put it in: Mac mini Late 2014 PCIe SSD Cable Replacement. If you have a blade SSD drive’d model then you have the needed PCIe cable but are in need of the SATA HD cable Mac mini A1347 (Mid 2011-Late 2014) Upper SATA Cable and heres the guide for that: Mac mini Late 2014 Hard Drive Cable Replacement If you have a Fusion Drive you have in fact have both cables present and you will first need to break the fusion drive set before you alter the drives How to split up a Fusion Drive

Yes with PCIe flex cable you can add nvme SSD card, and upgrade storage