Just bought a 4GB SD card today from Maplin, got it home and my camera (a Nikon D50) can't read it and displays an error (CHA), and my PC also cannot read it via a card reader. I tested the camera with another card and it worked fine, the PC was also fine and could read this other card.
Took it back to Maplin who were happy to exchange it, but the new card also has the same problem. Two dud cards seems unlikely, but possible, but could there be another problem?
The card is not locked, and when I try to access it on the PC (running Vista) the 'busy' mouse pointer appears and nothing happens, even after around half an hour.
4gig is big check with camera's paperwork to see if compatible with card that large and card readers can struggle. The reader in my 3yr old laptop won't read cards over 1 gig but the one in our HP photosmart printer will read my 2gig cards but never tried it with 4gig.