I rarely use DVD-RW/+RW (had more experience with CD, but that's another matter), so I didn't remember about having to pre-format +RW.
Right now I'm rewriting the +RW, and it didn't ask for reformat this time, fine.
That said, sorry about the -RW: actually it's another +RW, my mistake. Then it could have happen I erased it long time ago, putting it back to non-formatted state, here's why it had to be formatted again.
Well... all explained by my mistake above
Thanks to you both, now I know a few more things about DVDs
Great job with ImgBurn, and I love the music theme on finished operations