ojov

Try deleting BLUCHART.MPS before joining.

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My method to remove raster images from BlueChart G2 mapsets:

Example file - "Garmin VPC416S BlueChart G2 New Zealand North v11.50" downloaded from this forum.
gmapsupp.img - 520,822,784 bytes

Open GMapTool
Files > Add Files > gmapsupp.img

Split:
Directory > (select your folder for the ".img" files)
Create > single maps
Split selected

Delete or move the original "gmapsupp.img"

Close GMapTool (easiest method)
Open GMapTool
Files > Add Directories (your folder with all the now extracted ".img" files) > OK
Details

Look for any ".img" file containing the word "Bitmaps" on the last line and note them.

A part sample from my list:
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File: 08237056.img, length 9068544
Header: 10.07.2011 02:39:18, DSKIMG, XOR 00, V 0.00, Ms 0
xxxxxxxxxx
GARMIN G-Map
Copyright 1995-2010 by GARMIN Corporation.
CP 1252, Western European
Bitmaps 377, size 8461066 (4)
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Close GMapTool (easiest method)

Go to the folder and delete all the ".img" files with word "Bitmaps" on the last line.
DELETE "BLUCHART.MPS"

Open GMapTool
Files > Add Directories (your folder with all the remaining ".img" files) > OK
Join:
Output File > (I will call mine "D:\BlueChart G2 New Zealand North VPC416S.img)
Mapset name > (I will call mine "BlueChart G2 New Zealand North VPC416S")

Join Selected

Done - BlueChart G2 New Zealand North VPC416S.img - new size - 311,230,464 bytes