I have the Nikon D70S; very similar to the D70 except for the fact that the D70S has a slightly larger LCD screen.
Occasionally I have seen this error message and my solution follows:
I don't like the idea of repeatedly re-inserting the memory card. My solution to this is to leave the memory card inside the camera as much as possible. I have attached a short USB cable to the camera body and use this cable to transfer photos from the camera to the computer.
I leave this USB cable attached to the camera body. If not, I will have problems with the constant removal and re-attachment of the USB cable.
I found the problem that causes "FOR" and "CHR" to show and stop the camera working is dirty contacts from the card to the camera, I lightly sprayed electronic contact cleaner on the pins in the camera and the card sockets then plugged the card in and out 10 times, then make sure the card is formatted to "FAT" and all works, at least my has ever since.