I haven't tried this yet myself - has anyone else? http://www8.garmin.com/support/agree.jsp?id=3291
See the discussion here on Garmin's support forum: https://forums.garmin.com/showthread.php?t=27145
I did it. Froze my tongue to a flag pole too. ;)
With Win7-64:
I don't have multiple installs of MapInstall (Only one shows in the remove programs window).
The PC version didn't erase all the other maps in my Garmin folder, just added a new gmapsupp.img. Warned me when I overloaded the card and let me reduce the number of tiles selected. Doesn't seem a big deal.
Warned me I was about to overwrite a Garmin map.
I guess it'll send maps to the Garmin folder (wrong place) on my Nuvi as it did to my card, but the old version did that too.
What I still miss from MapSource is the list of tile names.
Add: Just sent a maps to my Nuvi 1490. The interface was really nice, BUT on my card I had a Map folder and a Garmin folder. In the Map folder I had a file called UT-AZ-NV-NM.img. It was a transparent overlay map. The Garmin folder was empty. After the transfer both UT-AZ-NM.img and DesertSouthwest.img were both in the Garmin folder and the Maps folder was empty. :( I couldn't see the maps on the Nuvi until I manually moved the two files to the Map folder. Then everything was fine, but they're going to get a lot of complaints if they don't issue an update pretty quick.
I've read about a problem with MapInstall sending to large cards. I'm using a 16GB card and that doesn't seem to be a problem.
Not sure what Multiple Map Files means, since I didn't try sending multiple mapsets simultaneously, but files had names of map products.