I think you can erase an RW disk but you have to use Disk Utility to do it. I think you select the optical drive and then erase disc is one of the options on one of the tabs... its been so long since I worked with RWs I don't entirely recall. Toast certainly can quick erase, which just wipes out the start of the disc and not the entire disc.
Well, that's what I've been trying to do, DU, erase RW . . . which does happen in the MBPro, but seemingly so far is not happening in the PM . . . .
As I recall, the R/W standard was never universally accepted, on Macs we had to erase them to use them again, on PCs they could just Erase the Directory & didn't need to Zero the whole Disc, later Macs got Toast with Quick Erase that did the same thing, & I think Dragon Burn does that too.
Apple never included that option in their SW... they were too busy turning their backs on customer wants/needs.
Also turning their backs on their machine progeny . . . but, I checked my OSX side in Tiger and I have CD Session burner from way back, from Panther days or actually Puma . . . but obviously that won't work on DVDs. So, I'm going to check out this Dragon Burn later on today or tomorrow . . . .