Norton 360 issues a message that it can not find the usb connedted hard drive, even though Windows 7

We have a windows 7 PC running Norton 360. The usb attached hard drivce is visibel and accessible from Windows 7. But the 360 back process issues a message that it can not find the USB drive. Until a few days ago, the same Norton 360 with the same back up parameters was working.

 

How do I ge t 360 to 'see' the usb hard drive for backup?