I assume that you've tried other cards to verify it's not the reader. That said, I've seen some issues of when the card was really full and not readable by the computer, but is by the phone. The fix is usually what Jeff mentioned, read it via the cable. It happens due to how the phone and the pc read the segments of memory from it. Also when it comes to clearing the data from the card use the phone's functions to format the card and you'll have better data integrity.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault