Welcome to CDFreaks!
Congrats on your Benq 1620. Though many people are snapping up 1640's with the recent sale prices, there are plenty of 1620's in use. Just check out the 1620 media threads, and you'll see it getting updated daily.
As for 'detection' of the blank media, are you referring to Windows Explorer showing free space? Or, does it not detect it when you insert a blank DVD for burning in your burning program?
If it's Windows Explorer, try seeing what happens in your burning software. If you get your compilation together and ready to burn it and it doesn't detect the media, then that's a different story.
What blank DVD's are you using? Brand, +/-R, speed rating, Country of Manufacture? Do you have Nero, DVD Info Pro, or Nero InfoTool that can identify the media you insert?
Also, which firmware are you using on your Benq 1620?
Did you have burning software installed PRIOR to installing your Benq 1620? Sometimes, the burning software and burner 'jive' better when you install/re-install the burning app after installing your drive.