Thunderbolt 3 to Thunderbolt 2 Adapter Not Working??

There is no gain in using the enclosures Thunderbolt 2 connection over using it's USB 3.1 connection.

Reason being, is that a USB 3.1 connection is plenty fast enough for the SATA 500MB/s SSD that enclosure accepts.

Just make sure that you have a USB 3.1 cable or adapter and cable, instead of a USB 2.0 cable or adapter and cable.

Connection design is not the issue as long as the port supports USB3.0 or higher.

Sorry for posting an image of a High Speed USB-A to USB-C cable earlier.

The OWC Dual drive has a High Speed 3.1 USB-B type port.

So your fine for speed, if you are using blue High Speed USB 3.1 to USB-A cable.

or a blue High Speed USB 3.1 to USB-C cable with the M2 Mac mini.

One possibility may not be the adapter but the cable being used. It is not obvious but cables can go bad even if not being moved at all once connected. Since you were using a TB2/TB3 adapter, I assume the cable is a Thunderbolt2 cable which has active electronics and can go bad.

As for the USB interface, you need to make sure the USB-A to USB-C cable does support USB 3.0 or higher. Many of these types of cables can be just charging cables which will only support USB2. As others have said, transfer speeds with USB3 (of which there are a lot of variations) will be more than fast enough with a SATA SSD in the enclosure and definitely fast enough for any spinning disk drive.

USB-C to USB-A cables with the blue insert are plenty fast enough for SATA 2.5" SSDs.

Marc Schreiber wrote:

OK, but I am using a USB 3.1 to USB-A. From what you are saying, the USB-A connection does not severely impact data transmission, either with a -C or -3.1 cable.

That is correct as long as you are using a USB 3.0 or 3.1 cable.