I want to say that I remember hearinb about a bug in SQL 2000 about backing up to a network share, but now I cannot find it. I have thought of a couple of other things to try though:

- Try mapping the share and then trying to perform the backup through the mapped drive (although that may not work if the service account cannot see the drive)

- Try the steps in this article. It is speaking about backing up from one server to another, so I am not positive that it will work if you are backing up to network storage such as a SAN/NAS. http://windowsitpro.com/article/articleid/14025/why-cant-i-backuprestore-my-sql-server-database-to-a-share-on-another-server.html