64GB cards work just fine in the Rezound, although if the file system is not the correct type (FAT32) it will need to be reformatted before use, and he phone doesn't have the capability of formatting it anymore (GB yes, ICS no). If it is not recognized and usable out of the package it will need to be formatted in a external device, like in a PC with a card reader, with a FAT32 file system. Also note that Win7 and newer OSs don't have an option to do that, so an external formatting tool will have to be used or an older version of Windows like XP.