Vista - Copying to Dvd-Rw

I backup my 'Documents' folder to D:, which is a DVD RW,
using 3 alternating discs .
When I right click on the 'Documents' folder and use Send To
Vista Home Basic instructs me to insert a writable disc. Which means
I must erase the disc first. But, on another disc I don't get this message
I get only a prompt as whether I want to merge the folder with the folder on
D:
First, why would I get different messages for different discs and second
is there a way to suppress the overwrite messages generated when copying to
the disc? Can Vista simply overwrite or erase the disc first?

James