just to clarify, your saying that your mdx is not reading your sd card?
have you tried setting >safely unmount sd card>remove sd card>turn the mdx off>take sd card off>take battery out for 1minute>put sd card back>battery back >turn mdx on>
now if your sd card has been properly formatted, it will automatically read your sd card.
however if it needs to be formatted, it will ask you if you want the mdx to format and say yes and you'll know when its done and you should be good to go.
btw, go to market download diskusuage by ivan volosyuk, it will give you more detailed info on what's using internal and sd card memories.
goodluck and let us knows on how it goes. :-)
English is my second Language