Well it will depend on the version of maps on your SD card. The option to display this is buried deep within the menu system. Thankfully GTRI01 mentioned it in a previous post, so here's a cut n paste of it.
From the map screen press menu > more > help > about > more > content > scroll down to maps
I also tried using HD Clone to clone my Subaru 4GB SD card to a SanDisk 16GB. The clone appeared to work, created the two paritions to the same size as the original SD card, however the unit failed to load, got stuck at about the 75% mark. May have something to do with the 3rd partition on the SD card, the unclaimned 12Gb of space. Will try removing that partition to see if it helps.