I have also purchased (Sept 2019) and installed a Samsung 860 EVO 500 GB 500GB 2.5" SATA III into my Mac Mini Mid 2010. The original system drive (5400rpm 320GB drive Toshiba MK3255GSX from apple) has been running really slow, like 4-10 MB/s read/write speeds. The link speed has also negotiated at 1.5Gbps so maybe even though the Toshiba MK3255GSX is Sata 2.0 there might be a bad cable. I installed the Samsung 860 EVO and booted off a High Sierra USB, and the disk utility can’t even see the SSD :( I’ve re-seated the cables, but nothing. Put the original drive back in and the system booted up fine, but is still super slow and negotiated to SATA 1.0 link speed. I’ve read all the other comments from Dan about swapping SATA cables; but I can only find “original” SATA cables for my SATA 2.0 capable chipset (based upon the screenshot 3Gbps link speed listed). If I’m going to find a correct cable for the default HDD on the Mac Mini Mid 2010, which one is it - that will work with my SATAIII drive (that link...