If you are using a SDHC to USB adapter, try another one if you can get one. Clean contacts. Worked for me when I had a similar problem. The sandisk adapter didn't work but a cheap no name one did.
I've ruled out issues with the reader, as I can use several other cards with it (including another exact same Sandisk one). I've also tried the offending card in another laptop and found it does exactly the same there.