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Your SD card might be corrupted, re-format your SD card
Yes,as suggested above,try to low level format your sd card.What card are you using?
I always suggested users to buy Sandisk or Sony micro sd cards.
I am not that sure,but there are compatibilty issues between some cards and the xperias.
As I have suggested above,in some cases a low level format has helped.But could you try other SD card?