Drive Imaghe XML error

I am using DIX to make an image of the system computer at my lodge. I creates a 4+GB image from a disk that contains 45-50 GB f used space. I run DIX from a batch file using the windows scheduler.
File follows;

rem Get Date Number
: DayNo %1
for /f "tokens=1-3 delims=/- " %%a in ('date/t') do (
set mm=%%a
set dd=%%b
set yy=%%c
)
set XDate=%mm%%dd%%yy%
echo [%XDate%]
"C:\Program Files\Runtime Software\DriveImage XML\dixml.exe" /bc /tD:\Driveimage\%Xdate%_Backup /r- /s- /c- /v

Note that I append a date stamp to the filename so I can save more than one image.
I am backing up to an external HDD that is gettings its data via a USB port.
Any ideas about why the smallimage.
Bill