I have a Nikon D70 and one day my camera said "the card could not be used" and I put the CF card into another dslr and it worked perfectly fine on that one. I only had five pictures on that card if that matters. Then I bought a brand new CF card and the camera said it needed to be formatted, so I formatted it on the camera but it still said the card could not be used. I'm thinking since it's an older nikon model that the camera is the problem, but I've had it for a while and had no problems before at all.
The CF card pins inside the camera might be bad. Frequent inserting and taking the card out is not recommended. I have a Nikon D70s and I leave the CF card almost permanently inside the camera. Even the firewire cable is connected to the camera and attached to the flash frame to prevent frequent unplugging. Soon after a photo shoot sequence I plug in the USB end to a computer and transfer the pics; make a backup, then delete the pics from the CF card in the camera.
My suggestion to you is to carefully use a can of compressed air to clean the CF card contacts inside the camera and get the connector pins working. Then insert a CF card and leave it in the camera. Do not take the CF card in and out.