Repairing a bad/damaged/corrupt CD card in Ubuntu Linux

This method uses fsck, the standard file system check application. First let's find the device path for your mounted SD card:
$mount
Find the card that represents your SD card. it should be something like "/dev/sdc1". Now, do the following:

$unmount /dev/sdc1
$sudo fsck -a /dev/sdc1

(where /dev/sdc1 is the device path for your SD card.)