Antilles already explained this. SDXC opted for Microsoft's proprietary ExFAT as default filesystem, so devices with which you can use a SDXC card have to support r/w support for ExFAT. Only then do they support the SDXC standard. Hence, if you just keep your card inside your 'N9' and its formatted say Ext3FS nothing is wrong. If you stick the card in your Linux laptop it will also work. But when you stick it in anything else (a digital camera, a Windows machine, a Mac machine) it won't work since these devices cannot read/write Ext3FS. Theres some hope here 'n there tho (Source) Unfortunately a lousy solution since it all won't work by default which equals to the consumer won't eat our dogfood but will eat Microsoft's proprietary ExFAT crap.