Backup to dual layer DVD+R Dl is supported. Copied from KB 943914 :
Back up to a disc burner
To use a disc burner as a backup device, you must have a disc burner and blank-writable discs. Windows Live OneCare supports the following writable disc types:•CD-R
•CD-RW
•DVD-R
•DVD-RW
•DVD+R
•DVD+RW
•DVD+R DL (Double Layer)
Note OneCare cannot use a CD or a DVD that has been formatted or erased by using a third-party program, such as Nero or Roxio. OneCare can only use clean or new CDs and DVDs that were not formatted by a third-party program.
The issue may be caused by a Nero packet writing application. I am not very familiar with Nero but Roxio has an application called Drag to Disc which runs in the system tray. If your version of Nero has a similar application, exit the application and try to run One Care backup. If you are still unable to run One Care backup please contact support. How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
