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...