DVD burning problem



I burned 500 MB data in a DVD using NERO and also checked “allow files to be added later”. But now, when I try to burn more data in that DVD, NERO does not burn it, a error msg occurs: “not able to write Disk-at-once on an open disk”. What is the problem. ID DVD not multisessional like CD. Can’t we make a DVD multisessional. I have bought BENQ DVD writer recently, so I dont know much about burning DVD.
Plz help me and give information about it.