Ghost reports EC8F0010 and 000003ED "volume does not contain a recognized file system"

About a week or two ago my nightly backup started failing with the error

 

---------------------------Backup Jobs---------------------------

Backup of C:\ did not complete successfully.
Error EC8F0010: Cannot track changes to drive C:\. Error 000003ED: The volume does not contain a recognized file system.
Please make sure that all required file system drivers are loaded and that the volume is not corrupted.Unspecified error

---------------------------OK   ---------------------------

 

I have tested my drive every way I can and I found no evidence of corruption or other errors.  Is there any way to tell what Ghost is seeing that generates this message?

 

The error started spontaneously - I have not made any changes to my system.  Although it did seem to start shortly after my NAV asked if I wanted to upgrade to 2010 (I declined).

 

I tried re-installing Ghost but that didn't help.  I tried deleting my old backup job and creating a new one but that didn't help either.  

 

If I do the backup manually (i.e, right-click on the drive and select Back Up Drive) it works fine, but the schedule backup always fails. 

 

I have Ghost  9.0 on Windows XP SP2.

 

Any ideas would be greatly appreciated.

 

Thanks!

Phil