Vista - Backup error in Vista Ultimate

Windows Backup gives me an error when I try to do a full backup of my c: drive. The error states "the disk in the drive has too little space." I'm using DVD-Rs (4.7 GB) in a DVD-RW drive. My c: drive is an 80GB partition with 24GB of free space. I can't get the backup to run. What's the problem. (I'm using Vista Ultimate 32)

You didn't select the DVD radio button

Windows Backup asks for a location and I click on the radio button DVD-RW (E). Then the system starts to save the back up before it writes it to DVD. There's no other place to tell it that these are DVDs. It actually asks for DVDs (assumes they are DVDs) since it requires the disk to be bigger than 1GB.

Any other ideas on why it tells me a 4.7GB DVD-R doesn't have enough space? The DVDs are brand new and unused.

Does it tell you how many DVD blanks you will need first? It tells me I would need from 21 to 36 DVDs then tells me to insert a disk with more than 1GB, but I never have actually tried it since I don't want to do a full computer backup just for the sake of trying it and wasting 40 DVDs.

Windows Backup can't use DVD-R's apparently. Only DVD-RW discs.

According to the "Help" files, any writeable disk can be used including CD-R and DVD-R.

Writeable disc
A CD or DVD disc onto which files can be copied. Common writeable disc types include CD-R, CD-RW, DVD-R, and DVD-RW.

This "writeable" popup definition found down the page here under Storage devices; Methods for backing up your files - Windows Vista Help is contradicted somewhat by

Obviously, you can't delete or change the content of a CD-R or DVD-R once it is written on this type of media.

Microsoft please make up your minds

Thanks everyone.