When saving your data, it is best to use your DVD or CD burning program to create a disk. Select the folders where your data is stored and burn it to a DVD. You can use a non-formatted DVD-RW and add data as needed. I would recommend you also make a permanant copy on a DVD-R. Formatted DVD-RWs or CD-RWs tend to have problems with reading the data later on or by other machines. I'd suggest you make another copy and see if the problem continues.
I backed up my files to dvd using the send to option for windows xp. Now I can't read the dvd. it shows no files but the dvd is full.