I tested out the effects of Sharpie markers on recordable media many years ago. Back then, I tested labelling CD recordable media and noticed no real differences between my CD-Rs that were not written on with a Sharpie felt tip marker (I only use the fine point ones) from those that were labelled with a Sharpie marker. This discussion came up the other day in a thread on the BenQ forum and made me want to test this out again but with DVD media.
I only tested out two discs because of a lack of time. Both discs were TDK DVD+R media (YUDEN000 T02). They also both had the exact same movie copied onto them. The first one was burned at 8x speed and the second one at 12x speed. This was to see if Sharpie markers made a difference at more than one burning speed. I first scanned the burned discs without any Sharpie writing on them. Then, I re-scanned them after writing on them with a Sharpie fine pont marker.
I conclude that contrary to my tests in years past, the Sharpies do negatively affect the discs; but only minimally so. The difference is nothing to worry about and both discs still scored very high in quality. Using Sharpies will not make your discs unreadable or anything like that. But, they do add a few errors to the disc. If anyone out there would be kind enough to test the same out, please post your scans in here. First burn your media without writing on them. Then, write on your already burned media with a Sharpie marker (fine point preferred) and re-scan them. Of course, keep the content (i.e. - movie) the same for all of your burned media and use the same media for your tests. Please do all of your error scans at the same scanning speed, in the same drive, and on the same PC system for all of your error tests as well. Feel free to test at more than one burning speed.
Here are my scans: