If it is still acting up, you could try connecting the scanner to your computer via the USB cable in Mass Storage mode.
Then open your Windows Explorer and you'll see the scanner showing up as a removable drive. You can right mouse click on that drive, then go down to properties and see how much used and free space there is. That'll tell you how full/empty the SD card is.
Following that you can, using Windows Explorer, navigate through the various file folders on the SD card and see if anything jumps out at you as far as being unreasonably large. There are two sub folders in the audio folder (inner_rec and user_rec). You may see some WAV files in them. There may also be some files in the Discovery folder if you've done any Discovery sessions.