Update mechanism
If you feel update is not working, that may be caused by camera managed modified file. Some camera manages special meta data file. This meta data file is modified from time to time.
If there are modified file, ND2901 may or may not consider the full card as a new card depend on the condition.

Check your firmware version.
If the firmware version is later than V1.04, it operate like following.

1) Modified file is NOT photo or video file and modified file size smaller than 512Kbyte, then it is updated.
2) Modified but file size is NOT changed, then it is updated.
3) If modified file is hidden or system file, it is updated.

Or if there are single modified file which does not fullfill above mentioned condition, ND2901 regard that memory card as new one, and does not Update it. Only Copy & Verify will be available.

Also, if you delete one photo and take new photo, then that new photo file may take the place of deleted photo position inside directory.
ND2901 checkes the file order to decide Update or not.
This inserted photo break the file order between memory card and HDD and not Update.