Here are the steps I use when I encounter issues similar to the one the op has.
1. try it with another computer.
2. If it does the same thing I'll remove it from the enclosure and put it into a pc.
3. If the pc fails to work with the drive internally, I'll rma it or destroy it. Note, opening up the enclosure may void the warranty.
If the drive does work in the pc, then I'd suspect an issue with the enclosure, which is not uncommon.