T3200M Router wipes all data on external drive

So I connected an external drive to the T3200M router and used it as a network drive by following this instruction (link) 

For some reason, the router wipes the external drive and all data on it is gone if I disconnect and reconnect the external drive.

It also happened sometimes if I reboot the router or reset the router to default setting.

Is there any ways to prevent this happening?

Thanks