SSD in external usb 3.0 case not recognized
USB 2.0 doesn't provide enough power for the SSD in an enclosure unless you have a case that has a plug where you may add more power (and that's going to be hard to find because USB 3.0 2.5" enclosures normally do not require such extra power and, therefore, will not have the 5v power port).

Or try a powered USB hub between the case and the Mac. That may work.