Retrospect State My DVD is Write Protected and Can Not Proceed

[hrdeutsch]

I am trying to add to my full backup on DVD-R discs. The last backup was disc #15 of the set. All discs are DVD-R from the same package. When I try to continue, Retrospect 7.5 on Windows XP asks me to insert Disc #15 into the DVD drive. When I do so it states that the disc is write protected and that I should remove the write protection.

 

There is no write protection on the disc. Does anyone know what the problem is and how to resolve it?

 

Should I ignore the problem and proceed to a new disc #16? Will that render the prior discs in the set useless as it will make the catalog unusable?

 

TIA,

Howard