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Info from Sony -
Notice for SDXC / microSDXC memory card and exFAT format used with Sony P & S Tablet.
Please be informed that Sony Tablet S/P series do not support SDXC/microSDXC memory cards, and exFAT format. When an exFAT formatted memory card (*2) is inserted in the SD/microSD memory card slot, your data in the memory card will be damaged. Please be sure not to use exFAT formatted memory cards.
Furthermore, data will be damaged as well when exFAT formatted device is connected to USB(Micro-AB) connector of Sony Tablet S series. (*3)
(*1) SDXC/microSDXC memory cards are preset to exFAT format at the point of purchase.
(*2) Besides SDXC/microSDXC memory card, Sony Tablet S/P series do not support other memory cards and other recording media that adopt exFAT format.
(*3) When a digital camera inserted with SDXC/microSDXC memory card (exFAT formatted memory card) is connected to main unit of Sony Tablet S series through USB(Micro-AB) connecter, your memory card data in the camera will be damaged as well.
I have installed a microSDXC 64GB and it works very well on my Sony tablet P using the FAT32 format (done via another android device).
Just in case anyone else is confused by this, here's some information from Wikipedia which will clarify the difference between exFAT and FAT32, etc.