> I booted, then tried to 'vi', but I get an error: Read-only file system. 
> I tried mkdir, same thing. 

When you boot, the root is mounted read-only; this is true of all ports, and
all disks.  To update the root to read-write, use the command "mount -u /".
(The "mount -a" in /etc/rc also accomplishes this.)

(Note:  EZ cartridges ALSO have a read-enable tab on the bottom, a little
silvery triangle near a corner next to the label.)