Montana 700 Crashes when trying to load IMG map file

I am having real problem getting IMG files loaded on to my new Montana 700. I have a 32G SD card and its FAT32 formatted on a Mac. I have loaded a IMG map file on to the SD card - a file that is 1.4G and covering the UK. 

Montana 700 does not see the IMG map file when I go into configure maps on the GPS.

So I moved the IMG file on my MAC into the GARMIN folder of the GPS and the IMG file is seen in configuration but when I select it and exit configure the GPS crashes - switches off immediately.

Really frustrating as I have the latest firmware install.