NSA221

Only found the manual from this link @ http://www.bunker.co.uk/manuals/ZyXEL NSA221 UG v4-00 2010-4-9.pdf
And you can download the " NAS Starter Utility" and follow the 2.5 Initialization Wizard from page 30.
Hopefully it will help you.
* This is for new start configuration, if you already have data in the HDD, I suggest to you do backup all of them to the external HDD before start *