external sd - "SD card is damaged - You may have to reformat." How do I fix this problem?

I had this before, formatted it and now it's happened again.

I suspect it's the problem mentioned here but I don't understand how to apply the fix from that thread.

I thought I had to download the file, rename it to update.zip, stick it on the phone's internal SD card, go into recovery mode and run "apply sdcard:update.zip" but I get an error when I do that which says:

E:signature verification failed / Installation aborted.

I'm not even sure if the fix is the right fix or the process is correct either, but if they are how do I get around this signature error?

It's a Galaxy S, running Froyo.