Is it possible to use this cable (LMP USB-C 3.1 to Mini-Displayport) to connect a Sandisk SSD (where the USB plug fits in) to a Mac Mini 2014 with its Thunderbolt 2 (whose connection is identical to the DisplayPort)? I would like to connect my fast SSD to this Thunderbolt 2 (because the existing USB 3.0 port is just too slow for large amounts of data - and I have seen the speed advantage on a new Mac Mini with Thunderbolt 3; there it is possible to connect the SSD in this way). But please only answers from experts here who have already made this connection in practice (and not because the connections are possible, so it should work - that's how I used to think). Thank you!

Question

Answer

Thunderbolt 1 and 2 can only transmit PCI Express and Displayport signals.
Only Thunderbolt 3 and 4 can also transmit USB signals and use the USB-C connection.

This cable transmits the DP signal that is output via the Alternate Mode of USB-C. This has nothing to do with Thunderbolt. 
A Sandisk SSD has a USB interface and it can only generate USB signals. For true Thunderbolt, it would have to be able to generate PCI Express signals; otherwise, the Thunderbolt port will simply switch to USB mode.

Basically, you can assume that Thunderbolt will only work if Thunderbolt compatibility is explicitly mentioned.
What you saw is probably the difference between USB 3.0 (5 Gbit/s) and USB 3.1 (10 Gbit/s).

Search

Question about LMP USB Typ C — DisplayPort