I tried to burn both DVD-R DL and DVD+R DL media on my (pre late) 2008 macbook pro with no luck. Toast 9 reported a “Media Error” and i also got an error when i tried it by burning a folder directly with Finder. After some googling i found a very easy, yet very successful fix:

clean your DVD drives lens 😉

Yes, it’s really that simple! I put an old and very cheap DVD lens cleaning DVD in my builtin slotloading DVD drive, let it spin for some seconds and then hit the eject button. After this cleaning Toast started burned my DVD+R DL disk without trouble. I will try it with DVD-R DL next.

Update: even DVD-R DL are working without a problem now!