first read
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FILES IN .SYSTEM folder:

4a) 006D135900.gpi (size: 1.159.063) I have no idea what this file is, so I'll leave it as it is - if you know pls tell me!!!
POI files - stored custom POI and travel guide files. These files are with .GPI extensions. It is ok to delete these files if you are sure you don't want them
4b) fs_image.ver (size: 14) same as above
sorry I donnow , leave it
4c) gmapbmap.img (size: 51.019.776) Since this is the base(global) map-I will leave it
correct
4d) gmapbmap.sum (size: 416) Check-sum for the base(global) map-I will leave it
You can delete, it's related at map update- not necessary
4e) gmapprom.img (size: 1.223.983.104) REPLACE WITH gmapprom.img (size:2.168.640) of the CT EU 2011.40 XT
correct
4f) gmapprom.sum (size: 416) - Since this is the check-sum file for the US detail map I plan to delete it, hoping that the s/w will rebuilt it for the EU map...
delete (same as above)
4g) gmaptz.img (size: 605.184) Time-Map, I will leave it.
Correct
4h) gmaptz.sum (size: 416) Check-sum for the Time-map, I will leave it.
You can delete
4i) Locales.xml (size: 71.846) I will leave it
Related to language and your position, leave it
4j) TrafficProvider.xml (size: 245.239) This file was _added_ after the last s/w update (step 1), I believe it has to do with TMC, I will leave it.
correct


5) I have some other maps from the EU 2011.40 XT SET. They are the following:
5a) gmaptz.img (size: 606.208) Perhaps I will replace the gmaptz.img installed in the Garmin because this file (of EU map set) is bigger (than the similar of US-installed) but I think it is not that important.
Probably the bigger one is an updated version, check with Gmaptool
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5b) gmap3d.img (size: 378.496) This is the 3D-EU map which will be installed in other GPS like the 755. The 1450 does not support 3d-mapping, I'll leave
Correct
5c) D2504030A.jcv (size: 681.441) Junction view file. I will put it in the folder .system\jvc after I will have deleted all the contents of this folder: D1279060A.img and D1279060.sum (US-junction view file and its checksum file)
Every release has a different JCV file, you can delete old maps ones,first backup on pc just in case. .sum files can be deleted.
5d) D2331040A.G2S (size: 308.421.696) Any idea what this file is and where to put it? (it seems it must be put in ".system/g2s/" folder)
about G2S read here:
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5e) SQLite folder
Already replied

8) If everything goes well, the next step will be to add an UNLOCKED Greek map with Greek fonts for use only in Greece
I will name the map gmapsupp.img and put it in the root directory [or in "Root\MAP\" folder with any name *.img e.g. GreekMap.img (ONLY for 1xxx units)]
Copy in Map folder, GreekMap.img
9) Next step to add Safety/radar cameras by copying respective files in POI folder (according to instructions of the respective file)
correct

good luck George