First off, your card/drive has a recycle bin for itself. If you can access the card by a card reader, right click on the drive( it will show up as a drive in My Computer ) and go to Properties. On he first tab, there is a button "Disk Cleanup". Click on that. It wil show you any files and folders that are taking up space. Clean out anything that you don't need, like the recycle bin. If that doesn't work, you may have hidden files that the computer is not picking up.
Go to My Computer, open the drive, then click on Tools-->Folder Options.
Then click on the View Tab. Choose 'Show Hidden Files and Folders'. Click on apply. See if there is a hidden files by opening the drives. Be careful as some of these files my be needed for the operation of the card.
Hoep this helps.