Netgear are currently offering to supply a free additional drive for the ReadyNAS Duo the size of the supplied drive depends on the version bought, as follows...
The No Drive version RND2000 qualifies for a free 500gb drive. 145.96 on http://www.dabs.com.
The 500gb version RND2150 qualifies for an additional 500gb drive. 176.97 on http://www.dabs.com.
The 1tb version RND2110 qualifies for an additional 1tb drive. - £258.75 on http://www.dabs.com.
Installing the additional drive will either double your storage capacity or ensure that your files are mirrored onto the second drive with the RAID protection depending on how you choose to set it up.
I personally will be RAID'ing mine to ensure that my Pictures, Documents and Media are safe from disk failures with plenty of room for the quadzillions of photos I have the habit of taking.
I will also now beable to access my files from any computer on my home network ideal for media streaming and infacrt my fist step for my HTPC setup although I still have no HTPC or streaming to the main living room TV, but all in good time huh!! Little steps :)
Check out http://www.netgear.co.uk/readynasduo.php?src=home
for the full benefits of a NAS.