Recently purchased a 64GB SD card and inserted it in the Zumo 660. My laptop sees both the Garmin (as E drive, and the SD card as F drive). I copied the "Music" folder from my pc with about 16GB of MP3's. When I go to the music player on the Zumo it says it can't find music. Also Garmin Express does not see the SD card. What am I missing?
It is documented. As for fixing it, either use a 32Gb or less card that can be formatted as FAT32 within your OS (although normally those cards would be FAT32) or find a utility that can format 64Gb cards as FAT32.
According to the FAQ Secure Digital (SD) card size limitations for automotive devices, the zumo 660 is compatible with up to a 32GB card. It may work with a 64GB card formatted in FAT32 as SUSSAMB mentioned but you may experience issues.
The reason that this is not documented in the manual is most likely because these limits can change with new types of cards being released quite often and new firmware updates on the device.