In searching the web a month ago, I wasn't able to find articles on using a standard M.2 PCIe NVMe SSD in the 2014 Mini. I was going to order a Toshiba/OCZ RD400 for some yet-to-be-defined future use NOT in the 2014 Mini (it's a good PCIe SSD that Toshiba will be discontinuing and the 256GB model was available at a good price). Before it's used in it's ultimate destination, I decided to try it out in my 2014 Mini, trying out High Sierra for the first time, and share the results. I'm happy to say that it does work. There's a thread in the MacBook Pro forum on using standard PCIe SSD's in the 2013-2015 MBP's that will be referenced in this post - I'll refer to it as the "big thread": https://forums.macrumors.com/threads/upgrading-2013-2014-macbook-pro-ssd-to-m-2-nvme.2034976/ Before proceeding, I should state that my opinion that I don't think a PCIe NVMe SSD in the Mac Mini is the preferable price/performance-wise choice for most people. On the Blackmagic benchmark, I get 688 MB/s write, 780 MB/s read (the one Blackmagic result I saw posted in this forum for the Apple 256GB PCIe SSD in the 2014 Mini was slightly, maybe 10MB/sec faster). The 2014 Mini has 2 PCIe lanes for it's SSD, the RD400 is a 4-lane SSD. So you're not getting the full performance capability of a PCIe x4 SSD. I also have a Crucial M500 (an older-model SSD) in this computer which has 414 MB/s write, 480 MB/s read. (I have thoughts on benchmarking at the bottom of the post.) The cost of the flex cable and adapter for me was $34. That's about half the current cost of a Crucial MX500 250GB SATA3 SSD. Probably any current 2.5" SATA drive will work in the 2014 Mini. However, if the 2014 Mini is like the 2013-2015 Macbook Pro, then there's a bit of uncertainty in just what PCIe SSD's and adapters will work. There are also OS and block-size considerations in using a PCIE SSD that isn't there for SATA3 SSD's. On the other hand, it is kind of geeky to do this so I won't begrudge people who do it for that reason. Selecting the Right SSD Before buying a PCIe SSD for the 2014 Mini, look at the big thread for the recommended SSD's. The Toshiba/OCZ RD400 (also known as the XG3), the one I tested, is one that's recommended. The Kingston KC1000 is also recommended. If you wish to get a different SSD than what's recommended in the big thread (with the risk involved), be sure to get a M.2 2280 (also referred to NGFF, 2280 is the size - 22mm x 80mm) PCIe NVMe (or AHCI) SSD. Do not get a SATA SSD, a M.2 2242 SSD or SSD's meant for the pre-2013 MBA. The SSD also has to be M-keyed. If you look at the SSD label-side up, with the connector towards the top, there should be one (and only one) notch in the series of connectors toward the right (you can image-search the Toshiba RD400 for what this looks like). SATA drives which use the M.2 connector will have two notches - these will not work. If you plan to use the SSD with Sierra, you need a SSD that uses a 4K block size. High Sierra can use 512-byte block size SSD's (more typical) or 4K SSD's. The RD400 comes with a 512-byte block size which can be re-formatted to 4K. The other Toshiba drives (XG4, XG5) should also be able to use 4K. The big thread also says the Kingston KC1000 and the WD Black can use 4K. If you get one of these drives and want to use the 4K block size, you need to find out if it comes standard with the 4K block size and how it can be reformatted if it isn't 4k. For the RD400, Toshiba/OCZ has a utility that requires the creation of a bootable Linux USB flash drive which contains their SSD utility software - I tested this and it works on the 2014 Mini. OS's before High Sierra will not see a 512-block size PCIe SSD so you can't use the HS installer running on an OS before HS. A 512-block size SSD is visible to a bootable High Sierra USB flash drive installer. I don't believe you can use non-Apple PCIe SSD's on El Capitan without installing a kernel extension - the big thread may have more information on this. The big thread SSD recommendations is based in large part on how well it performs in the 2015 MBP, which is x4. It may be that SSD's that are not recommended will work OK in the x2 Mini - but this is speculation on my part. The Adapter For those that are unaware, Apple uses a non-standard PCIe connector in the 2014 Mini which requires an adapter to work with standard PCIe SSD's. The most popular one seems to be the Sintech green "short" adapter available from Sintech directly at: http://eshop.sintech.cn/ngff-m2-pcie-ssd-card-as-2013-2014-2015-macbook-ssd-p-1139.html I bought mine from eBay - you can search for "Sintech M2 M-Key SSD as 2013-2014-2015 MACBOOK SSD" to find it. In the big thread, this doesn't work for all people. Some people in the big thread also needed to use Kapton tape (used as an electrical insulator). I did not have to use it. The Flex Cable Unless your Mini came with the Apple PCIe SSD, you will need to buy the flex cable. Search for "821-00010-A SSD PCIe flex cable" to find it. I bought this on eBay and all of the ones I saw from different sellers seemed to be the same cable. The Screw If you didn't get the 2014 Mini with the Apple PCIe SSD, you'll likely need a screw for the SSD. You can check your Mini to confirm this. If you look at the iFixit teardown: https://www.ifixit.com/Teardown/Mac+Mini+Late+2014+Teardown/30410 you can see the screw in the step 9 of the teardown towards the left side of the picture. I was lucky in that I already had another computer which had an empty M.2 slot with a screw. You can search the web for "m.2 ssd screw" for places to buy the screw. The standard size seems to be "M2 x 3mm", but my screw was 4mm. I would suggest the 4mm size as there's enough room in the screw hole for 4mm to hold the SSD down properly. You should get a screw with a "wafer" head. You should not use the PCIe SSD without a proper screw. (If you're thinking about it, no, the screw used to mount 2.5" drives will not fit.) The Plate If you look at the iFixit teardown link, you'll see that there's a metal plate held down by two screws above the flex cable logic board connector. None of the flex cable kits had this plate and I did not use it so it doesn't appear to be necessary. Installation Look at the iFixit page for the PCIe SSD cable replacement: https://www.ifixit.com/Guide/Mac+Mini+Late+2014+PCIe+SSD+Cable+Replacement/32652 Follow the steps 1-5 for removing the plastic cover and the antenna plate. In my opinion, removing the antenna cable is not necessary as long as you are careful in handling the antenna plate. Steps 10-17 cover removing the flex cable so should be used for reference, not actual installation steps. The flex cable has two holes on the side of the cable connecting the SSD. There are two pins on the surface of where the SSD will go that will match the two holes (take a look at steps 14-17). Keep the peel-off adhesive protector on and place the connector in place to get feel for how it will fit in the Mini. Notice in steps 13-17 that the cable has a slight bend in it. These show a bend down in the cable. I did a bend up. Although it appears in the pictures that a crease was made in the cable, I wouldn't crease the cable. It should be stiff enough to hold a bend without a crease. Once you're comfortable with how it's going to fit, you can remove the peel-off adhesive protector and place the cable in place. I attached the Sintech adapter to the flex cable before putting the cable in place. Attach the logic board connector - do not use too much force. Attach the SSD to the Sintech adapter and put the screw the in. For me, the SSD easily slid into the Sintech adapter. What I Have and Haven't Done The 2014 Mini is not my primary computer - it's mainly used as a DVR and for video transcoding. I also had to do some tests using the latest Xcode version (5GB download) which requires High Sierra. My Mini also has 50GB of audio samples used for music synthesizing. So I put a decent amount of data on the SSD. I've used the PCIe SSD in the Mini for about a week now without any disk-related issue. The PCIe SSD is located close to the WiFi antenna. I did not have any issues in this regard. This Mini uses WiFi for network connectivity and I spend more time screen-sharing to than actually working directly on the computer. I have not tested Bluetooth. In the big thread, people have issues with sleep and hibernation. I do not use either so I did not test this. I did not have the reboot problem mentioned in the big thread. A Note About Benchmarks Blackmagic is commonly used by end-users to measure the speed of their disks. It's a simple benchmark measuring a complex system. However, you'd be hard pressed to find somebody who runs and publishes comprehensive benchmarks doing a benchmark of a x4 PCIe SSD in a x2 system, much less the 2014 Mini. So in my opinion, having the Blackmagic benchmark results from the 2014 Mini, as limited as it is, is better than relying on the published benchmark results for a x4 PCIe drive run in a x4-capable system if you're trying to figure out how it will perform in a 2014 Mini.