Adding more back up data files to DVD-RW disc

I have some back up files into a DVD-RW, but cannot add more files, just dragging into the disc icon.

How comes ? and how to do it ?

Thanks for clarification

Well you didn't really make your question clear. But I'll take a shot at it.

If the DVD is blank than it shouldn't be a problem. If it has already been burned then you need to erase it first.

Like zeppster said you will need to copy the contents of the DVD to your hard disk and recreate the DVD with its new contents.

If I understand it right on the PC you are doing the same thing but you don't know it.

As previously stated, you can't write sessions on a dvd-rw, but you can write sessions on a dvd-r. (toast and Disk Utility)