Ghost 15.0 error message when trying incremental backup

The error message I receive when attempting to do an incremental backup is this:

 

-Unable to create incremental recovery point.
--Error E0BB0061: Win32/Win64 API [WindowsComputer::MakeSnapVolumeWriteable]  DeviceIoControl( FSCTL_LOCK_VOLUME, \\?\SymantecSnapshot1 ) failed.
---Error EBAB03F1: Access is denied.

(UMI:V-0-187-131)  [end]

 

I'm sure there have been other messages in the past but this is the last one I saved to clipboard.

 

My only solution to date has been to rename the Norton Backup folder (on my external drive) and let Norton create a new folder and do a complete backup.  Suggestions?  I'm using Windows Vista Home Premium and am up to date with Windows Updates.