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 negotiates as per Samsung Spec) on a SATA 2.0 controller?
So far - no working Samsung 860 EVO on my Mac Mini Mid 2010; and looking into firmware updates but it’s not so simple when the Mac Mini is my only machine with the Optical Drive.
Edit 27 Oct 2019: after replacing the SATA Cable with a SATA3 one specced for a Mac Mini 2012, diskutil command line showed the SSD, and then it was able to be formatted by Disk Utility and a restore of time machine kicked off.