I also have the same problem with my 4GB SDHC card. This is what I found:
"SD and SDHC compliance
If your computer's internal card reader works with 2GB (or lower) SD cards but fails to read a 4GB (or higher) SDHC card, then the computer's internal reader is not SDHC compliant.
This same logic also applies to digital cameras, if the digital camera reads 2GB (or lower) SD cards but fails to read a 4GB (or higher) SDHC card, then the digital camera is not SDHC compliant.
SD and SDHC specifications
The initial SD card specification defined the interface for cards only up to 2GB capacity. The subsequent SDHC specification was developed to provide increased capacity of 4GB – 32GB cards. Computers and other products purchased before 2008 may not be SDHC compatible. Computers and other products purchased before 2007 are not SDHC compatible.
Verifying the card reader slot
If the card reader slot on your computer does not have the SDHC logo, it is most likely not SDHC compatible and will only read cards up to 2GB capacity.
Other options you can use to read files on your SDHC card
If your computer's card reader is not SDHC compatible, you have 3 options available:
1. You can connect your camera (or other device) that works with SDHC to your computer using a USB cable that came with the camera. You should be able to transfer files to the computer this way.
2. You can contact you computer manufacturer to see if they have a free firmware update for your model that will make it SDHC compatible
3. You can buy inexpensive SanDisk SDHC card readers that will connect to your computer's USB port."