SDXC Card Reader with LPC1822
64 GB SDXC card, it gets enumerated and is displayed in the windows explorer as an unformatted SDHC card with 32 GB [...]


Check the USB Mass storage code. There might be another limit.


  [...] simplified spec. And this does not say anything about SDXC


Download an updated copy from sdcard.org. Newer versions have infos about SDXC in them.