Jon,
You are getting the I/O error because the filesystem is mounted read only (ro). If you remount the filesystem read/write (rw) you should be able to copy the file back. You may need to run fdisk to find out where / is mounted. Just type fdisk /dev/hda (if it's ide) then "p" to print out the partition table and q to quit, once you know where the root filesystem is do the following:
#mount -o remount,rw /dev/hdaX
(X being where root is mounted).
HTH,
Lloyd