Dennis' computer has a Western Digital Passport external hard drive that mounts, but it can't be read it or ejected. Then it crashes the computer. Leo says that it's a USB device that isn't fully mounting. It's probably an issue with either the USB controller on his computer, or the cable itself. Dennis should try using a different USB cable first. Then he should try plugging it into another USB port and see if he can replicate the issue. The USB driver may also be corrupted.
Another thing he can do is take the hard drive out of the case. Then he can use a temporary connector to test it. NewerTech has one for $30. Or he can just install it into his tower. If he can then see the data on the drive, then it's the USB enclosure and he can always just replace that. They're cheap. In fact, he should probably just do that. Conversely, he could put another hard drive into the USB enclosure and see if that works. Once he's eliminated the USB interface as the issue, then he can work to use a drive recovery utility like SpinRite to repair it.