try this. copy the data off the card to your PC.
then put it into your JM4 and try to format it. did that work?
if not, if it a normal kind of SD card? and not a mini SD in an adapter?
only up to 2GB cards will work, as anything bigger is classed as SDHC which is a different formatted card.
if you still have no luck, I'd say it's a fault with your JM4 and you'll need a service http://line6.com/service_centers/
+1 to Rowbi's recommendations
Also, have you tried using a different SD card that is 2 gigs or less? OR format the card you have on your computer to FAT or FAT16 format after removing the files as Rowbi mentioned.
If the problem persists, you may need to take the unit to an authorized service center: