Check with the reader manufacturer or your computer manufacturer for an updated card reader driver that supports the high density SD cards.
I had the same problem with a Compaq Laptop v2000 series with a built in card reader for SD/MS/Pro/MMC/SM/XD cards. My 1GB SD cards read and write just fine from my camera, but it would not read the 2GB SD cards I had just bought and used on my Caribbean Cruise that I had just returned home from. I was able to download the pics direct from the camera using the USB cable that came with the camera. I then did some research today and found an updated card reader driver from my computer manufacturer which added support for SD 4GB high density cards. Once I updated the driver, the 2GB cards read and write just fine now.
If you inserted the card into your reader and it told you it had to be formatted, and you did it.... you may have lost your photos/files in which case, they may be gone unless you try a recovery program such as those available from cnet.com. Safest way to make sure the photos are still on the card is by reinserting it back into your camera and playing the images back. Then, if you have a cable for your camera, use that first to transfer the images to hard drive or CD/DVD. If you don't have a cable for your camera, get one, they're not very expensive, and handy to have, especially in situations like these. Then, later, when you have an updated reader driver, try the card in the reader after updating and rebooting.
Good Luck.
Joe
