|First have you tried different USB ports?|
Sometimes over time a USB port can through regular use become a bit flaky in its connectivity. Using a different USB port can fix this problem.
Have you tried a different USB cable?
I assume it is in a caddy was it dropped or jarred?
You could try a different caddy. They are fairly cheap to buy (e.g. on eBay).
☺ When everything else fails, read the instructions.