I have Norton 360.

DVD is actually a Licensing Authority which holds the patents and does licensing for the people who invented various aspects of DVD.  This is relevant because UDF 2.6 onwards (that would be the DVD filesystem) allows not only discs recorded in multiple sessions but updates including writing the same file with updated content...as long of course as there is room left on the recordable media.

  What are you reading on the Memorex box anyhow?  I notice ' Re-Writeable' on DVD-R and +R all the time, though of course they're not rewriteable through normal ; maybe there's some thing where you they process them in an oven and reuse them, but it's unconsistent with the +R standard, and you should definitely double-check things at their website.  DVD+RW and +RAM have to be ' erased ' before they can be reused by doing a burn of that variety.  I am pretty sure you just need to buy more DVD+Rs or -RW or -RAM, and not expect to reuse +Rs you bought for 20 cents each.

     The most important thing here of course is that you've given the wrong category; you should use or create one for Norton 360.