External SSD with OS installed not working?
If it worked fine for several months then I would suggest the case supports it. It's very possible that the control board inside the case or the cable is damaged.

If you have the technical know how, I would suggest placing the SSD inside the iMac. That way you also avoid the bottleneck of having to go over USB. There are plenty of online guides on how to do this.

I have used several of the MacSales kits, all the iMacs are still running and it took me 30 minutes at most.