method used on SP2720
prepare a small gmapsupp.img with a map you don't need
rename to GMAPPROM.IMG
install Garmin usb-driver
install WebUpdater C:\Garmin\WebUpdater\WebUpdater.exe
copy the unlocked file to C:\Garmin\GMAPPROM.IMG
run update.cmd [erasing is done twice !]
After this you have free memory to load map directly with MapSource, and replacing it easy every time again with a new one just using MapSource.
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But if you just want one-time update you prepare a big map in step 1.