1. Many of us prefer using the uSD card for our maps and other data because, if you have any issue with data becoming corrupted or not otherwise properly read in the field, you can simply remove the uSD card, reboot the Montana, then insert the uSD card and reboot again to re-index all of your data. This does not happen often, but it does happen. 2. I have used many brands, and have had the best reliability with SanDisk cards. Officially, the Montana accepts up to 32GB, while some users have been able to make 64GB uSD cards work. Due to map tile and total GPX limitations of the Montana, 16GB will likely be plenty for most users. Map information here. 3. The most current version of Garmin MapInstall will load all map data to your uSD card before sending to your GPSr, but data transfer through the GPSr can be very slow. If you create a \Garmin\ directory on your uSD card and insert it in a card reader, MapInstall and BaseCamp will interface with it and data transfer speeds will be better. More useful software information here. Make sure you back up your Montana before getting carried away, just in case you need some of the original files date!