It sounds like the partition that it needs to use is mounted read-only. You can see how they are mounted by just typing the 'mount' command. If it is
mounted read-only (ro), then you need to remount it read-write (rw). Something like this:
mount -o remount,rw <partition> <mount-point>
If that partition is set to mount read-only at boot, you'll have to do this every time you want to write to it. It would probably be easiest to change the /etc/fstab file so that it's mounted read-write at boot.
Hope that helps...