The suggestion is to format the card in the camera.
This is weird. I bought a Targus memory card reader (FYI to replace one made by Digital Concepts that failed to note it only read cards up to 2 Gb). The first time I used in (MacBook Pro, OSX 10.X) it showed up, I was able to download photos to my iPhoto library, all is well. Then, the next time I plugged it in, I got a message that the drive was not formatted, did I want to format it? Disk Utility didn't know what to do with it and locked up when I chose erase. When I plugged it back into the camera (Sony A100), it asked to be formatted and do so in about 2 seconds. Now, when I plug the reader in, with the same card (with three test photos), nothing happens at all. No recognition in the finder whatsoever. What could be the problem? Further info: I get the exact same message using a Dell laptop (Windows Vista Ultimate). Desperate for help, thanks in advance.