Drive not recognized , intermittent problem

On Windows 7 PC, WD 1T USB 3.0, has been running without WD SmartWare Installed.   My Cobain Backup was running fine, then one day I noticed an error in the log - it could not find the destination directory.  I could still see WD drive under My Computer but all the folders appeared to be empty.  Eventually I could not see the drive at all.  Connected WD drive to laptop running XP and it was fine.  Reattached it to Win7 PC and it worked fine.  After a few days and some successful backups the problem reoccurred.  Some times when it reoccurs I cannot see the drive at all under My Computer, at other times I can see the drive but the folders  are empty.  I have tried many things: computer restart, connecting to another USB port, …  I only have success if I disconnected the WD drive from the PC and the power source and then reconnected them.  It just happened again today and was resolved in the same way.  I didn’t even have to restart the computer.   Any suggestions as to what might cause this?  

Some of the files in the hard drive might be getting corrupted, you can try running a test with DLG, if the test completes successfully backup all the files and then format the hard drive.

I downloaded the Windows Data Lifeguard Diagnostics and run it against the WD My Book and found no problems.  Why should I reformat the hard drive (I assume you mean the WD My Book) if there are no problems?