I also have Lumia 520 and my SD card is generating these messages in the last month. For the first time, I began to ignore them.
The message is caused by some incomplete information about the completion of file transactions. For example, the card stopped working at some moment and you rebooted the phone in the middle of some operation etc.
Most of these transactions don't affect any file you would care about. It was probably a cache file that became irrelevant quickly etc.
If you want to remove the message, turn off your phone, extract the SD card, place it into an SD card reader on your PC in the "write" mode. Run a command prompt as an administrator (right-click at the Start button and choose the Command Prompt as administrator).
In the shell, type E: or whatever is the right drive for your SD card, enter. (The command "dir" shows you the files. Search for MS-DOS to learn other commands.)
Then you write the command
press Yes, if asked whether you want to disconnect processes on the SD card, and run the chkdsk program. For me, it takes half an hour. When you release the SD card from the reader again and return it to the phone, the error message will be gone.
I've done it some 10 times in the past – especially when Mapy.cz, a great Czech mapping app, was creating too many small files and the SD card suffered and froze sometimes etc. Recently, the SD card crashed without good reasons, too. I have ignored the message for a month and it's causing no pr