I can confirm what Loly's saying, as mentioned in Post #5 i have a 260W HWID 0827. However, mine has been flashed with the standard 2x5W logo to enable the Traffic feature but that should not have made any difference to it's ability to use a large SD card [if you want to make your 260W into a 265T or 465 truckies' nuvi see this thread]: [Only registered and activated users can see links. ]. SF's is a 250W HWID 0827 but conversion to 2x5W/465 will work the same for your 260W 0827 unit.

Do you have the same problem either using your reformatted 64GB card in another unit, and also have you tried a small [say 4 or 8 GB] card in the 260W with correct placement and naming of map files? If not please try.