SSD in enclosure does not mount on older MacBook Pro
The answer is, make sure that your SSD has adequate power.

Some USB cables come with two USB plugs at one end.

The second and thinner part of the cable is needed for power supply.

So plug in both ends one in each USB port of the computer and then it should supply enough power.

Alternatively, and this made for a more reliable power supply, one can use any USB device that supplies power. In this case an iPhone charger attached to the second USB end (power-only) did the trick.