I wanted to burn a dual-layer DVD the other day, but confusingly Toast quoted the space available as 4.21GB, where of course it ought to be 7.92GB. This was the case regardless of whether a disk was inserted. Much playing around eventually found the cause, and a solution.
It turns out that one of the disks on a spindle of blank DVD DL (DVD+R, though I don’t think that matters) was faulty, and indeed only had 4.21GB available. Unfortunately it seems there’s a bug in Toast such that it then stores indefinitely this incorrect capacity as being what a DVD DL can store, preventing one from using the full capacity of a DVD DL thenceforth.
The solution to this is to delete Toast’s preference plist, that’s found at /Users/<username>/Library/Preferences/com.roxio.toast.plist. Once this is done and Toast it restarted, it’ll take you through the ‘first time launch’ spiel (but won’t lose your serial number registration) and, once you’re done with that, DVD DL capacity will be correctly reported once more.