The Topo maps are on a DVD. Windows and Mac compatible.
The Legend only has 8 mb of flash memory. You can download what you need of the DVD maps (I think they are broken up in county-sized sections. You choose which sections to load onto the Legend). As far as I know none of the Garmin GPS units generally available will download the entire US map all at the same time. That would require upwards of 4 gb of space. Even my new Legend which accepts micro SD cards can only accept 2 gb at a time.
You will not have space on any GPS that I know of the download the entire CDT’s worth of topos. You will need computer access occasionally to update the GPS with new maps.
You do understand that the actual map isn’t necessary to take advantage of the GPS, especially if you plan to carry paper maps as well - as you should. The primary use of the GPS will be to intermittently get a fix on your exact location, then find it on the Map and go from there.
Hope this helps.
lyle