I have a backup for M400 (it doesn't have SD card slot btw). Do you have USB connection with your PC if you connect your device? If yes, than you don't need firmware re installation..only unlock script along with the necessary files.
ps:
that link @catymag posted is for firmware not for the backup.
here download this package:
[Only registered and activated users can see links. ]
unrar and copy all files and folders to your MIO M400. Reset device and witness the miracle..
..ad maps, pois etc..
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