Help, files not found

I have a My Book Essential, 2TB, Firmware 1.007, SmartWare 1.5.1.6 with W7, SP1. Intermittently, My Book files become unavailable. The drive is in My Computer, and the files are listed, but if I try to open one, I get a message saying the file is not found. Also, the Set up Drive button  in Smartware is grayed out. The problem goes away if I reboot… at least for now. Any help would be appreciated.

Lou

As I feared, restarting no longer solves the problem. In fact now the drive does not appear in Windows Explorer at all. In Device Manager, there is an unknown device with the message: Windows has stopped this device because it has reported problems (Code 43).  As I said, any suggestions would be greatly appreciated.

Lou