Got the reply...did not address the SDSX 64gig question...did say that any brand as long as it was high speed card and a full format is performed there should be no incompatibility/card not recognized issues...and that he is using SanDisk SDHC Class 10 80meg Read/Write in his unit.
I've got a new SanDisk Ultra SDHC UHS-1 Class 10 32gig 80 mg/s 533x speed card in the retail packaging to try...we will see.