Nero 7 / Corrupt DVDs

I’m using Nero 7.7 on Windows Vista. I burned lots of "dvd-r"s recently. I don’t use verify option. (I use now). Anyway, When i tried to clone some of that dvds i burned, nero said it cannot read data.
Windows explorer displays file index, but cannot copy to Hdd too.
And when i looked at the dvd surface, i found out that almost all that dvds i burned has blank areas on them. Took a picture here:

And these shots are from Nero Infotool:

Now, what shall i do? Change the media? Or even change the dvd writer? I mean, i cannot deal with corrupt dvds. This time i was lucky, i did not delete that stuff from my hdd after burning copies. But i cannot trust this config anymore.

Any ideas?