Format for CDs/DVDs

I need some help backing up to DVDs:

1. Am I correct in assuming that it is necessary to format the blank DVD before use?

2. Under Windows 7, CDs/DVDs can be formatted either "Like a USB flash drive" or "With a CD/DVD player (Mastered)." Which format is preferable for backup DVDs?

The reason for my questions is that I pre-formatted some DVDs (using the "Mastered" format"), and succeeded in backing up and performing a restore, but when I tried to perform a second restore the last DVD in the set had become blank.

I would be grateful for any comments.