Need Help mylotters having problem in memory card...

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My ph.is N72.I have memory card of 512MB.And memory card is mostly full,the left space is about 12MB.FRom few days having problem in ph.gallery.when i open gallery to view any pics.or songs the list is empty.It shows there is no images and no songs.But when i check my memory card details it shows the MB used by songs and images.So today i insert my memory card in my laptop and taken all backup of memory card contents.I have 1169 messages in my phone memory..So aftr taking back up of memory card i formatted it and then i insert in my ph.and then i changed my message memory and transfered all my messages to memory card (so if sometime there is need of formatting my ph.memory my messages are saved).After tht i check my memory details so all total messages size is some 1.29MB and in memory details it is showing 22MB used.And so i try to change my message memory again to ph.memory then i cant able to change,it was not changing only.Then i remove memory card from my ph.and tried to change message memory to ph.then also it doesnt change.So i again formatted my memory card and inserted in ph.Then i tried,still can`t change.So i tried to open inbox but it shows Inavlid memory card.Uable to save messages.I tried many times but my inbox does not opened.So after i jst tried to back-up my ph.memory and aftr tht automatically my ph.memory changed but wht should i do for this memory card problem.