In addition to abrody's always good advise, you may be suffering from one other thing.
You say you have used the same media with NO problems many many times. If by chance you were burning Video TS.
You will find if burning a lot, your computer stores all these folders on your hard drive and will virtually not burn. Your explination is almost text book in having full cache 's
Give this a try.
Download , The Freeware Shareware, and run them
Macjanitor.
http://personalpages.tds.net/~brian_hill/macjanitor.html
Cache Out X
http://www.versiontracker.com/dyn/moreinfo/macosx/14850
Onyx.
http://www.titanium.free.fr/pgs2/english/download.html
Run all 3 and then try again.
If my quess is correct your problem will go away.
If so you will know after about every 3rd burn you will want to run these 3 pieces of software and you are good to go again.
This problem is very prevelent in older Optical Drives
Cheers Don