Nuvi 500, California and Oregon Topo maps. using Basecamp. Windows 7/64.
FYI, I had to format the SD card before the maps would be recognized. Prior to formatting the card, Basecamp downloaded the maps and they showed up on the card but the Nuvi could not read them.
All of Garmin's units require that the data cards be formatted as FAT32 to be recognized. Some larger cards may be pre-formatted as NTFS, so that might have been your problem. Another cause could have been a card formatted for MacOS.