I sometimes get downloads, usually of linux OS's where the file is too large for a cd and has to be formated as a dvd. When I try to burn it to a disk, I keep getting something that says I need to insert a blank dvd. Problem is, I have a blank dvd in the tray all ready to go. The unit I'm using is a Sony external dvd burner hooked up by a usb. Bresero wouldn't touch it, so I installed k3b (this is in Ubuntu10.04)
k3b could read that I had a blank dvd in the drive, but when I pointed to what file I intended to burn, I kept getting the "insert blank dvd", and it wouldn't start, even though the screen in the background could see that the dvd was loaded and ready. No problem for cd's, they'll burn like an oil slick on the Gulf of Mexico.

Just a shot in the dark, have you tried with different DVDs? It sounds like the DVD you loaded had some kind of error to me... Do your DVD burner read DVDs ?

I will second what Hammett says. A DVD+R may not work with many devices for example (they are particularily troublesome in my experience). It may be due to the particular brand of DVD not being compatible with your hardware.

Right click on the drive in K3B. Try to eject and then reload. Select properties on the disc and see if it says the disk is empty and writable.

Not all DVD discs are created equal (maybe due to legal patent reasons).

Maybe your user doesn't belong to the group "cdrom". Run the "groups" command and see. If not, then run "addgroup user cdrom".

Otherwise, I agree that the most likely cause is a problem with the dvd. Try another dvd and see.