Mass storage mode is not needed and is NOT used for all operation that you mentioned.

For changing preloading map you must use updater.exe with special key for writing certain region. Read forum threads for 276, StreetPilot or other old fashioned Garmin device.
Other map is installing by Mapsource into memory card only. Memory card have proprietary Garmin format and there are only two way to write map on it:
the first, insert card into device and use device USB port,
the second, buy not cheaper Garmin card reader if you want write map without devise on hand.

For FW you must use the same updater.exe with downloaded files from garmin.com. But for me dashboard.com work fine too. Garmin Webupdater is present too.

For all you data (tracks, points, routes) you must use Mapsource, Basecamp or other Garmin compatible programs for USB port. Also you can use very slow COM port on device at all classic fashioned GIS and PC.

Mass storage mode is only for direct operation with files but this device do not have such drive to say again...