problem adding grids in gdedit

[Case.Jon]

Dear gembud,
 
I recently tried to add a grid to an existing GEMPAK file using gdedit, but I receive the following error:
blinky:/projects/proto/WRF/EMS/wrf/runs/CONUS/wrfpost/gempak > gdedit
 GDEFIL    Grid edit file                   gdedit_cbas.fil
 GDFILE    Grid file                         200608172100_nmm_pres12km.grd
 GPACK     Packing type/number of bits        
 Parameters requested: GDEFIL,GDFILE,GPACK.
 GEMPAK-GDEDIT>r
 [GDEDIT -6]  Error reading grid data.
 Parameters requested: GDEFIL,GDFILE,GPACK.
 GEMPAK-GDEDIT>e

A little background:
Using the existing GEMPAK file listed above, I ran gdlist to create an output of an entire grid (601x371).
This grid contains negative values that I want to set to missing (-9999.00), so I substituted the negative values using shell commands, preserving the row and column orientation, and wrote the data to a file called "gdedit_cbas.fil".  When I tried to add the new grid to the same GEMPAK grid file, I got the error above.  
I have ensured that the file has the same # of entries and rows as the actual grid file.
 
FYI, the header of "gdedit_cbas.fil" is as follows:
 
 Grid file: 200608172100_nmm_pres12km.grd                                       
 GRID IDENTIFIER: 
    TIME1             TIME2         LEVL1 LEVL2   VCORD PARM
060817/2100F012                         0                   NONE   CBAS        
 AREA: DSET                                                GRID SIZE:   601  371
 COLUMNS:     1  601     ROWS:     1  371
 Scale factor: 10** 0

 COLUMN:      1        2        3        4        5        6        7        8
(etc, etc, etc)
 
Your feedback & assistance is greatly appreciated!
Regards,
Jonathan Case
 

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