08-30-2010 06:59 AM
Most likely your adapter is too old to support the SDHC format, which anything above 2GB is. I'd get a new, more current one.
Also realize anything you get today will not support to new, soon to be popular SDXC format (>32GB), so you'll eventually have to get another one if you plan on using those cards. That may be a couple years down the road though.