Is this a regular Windows policy, or is it a flash backup? (there are some possible tech notes on flash backups)

What is the backup selection (path)? (are there excludes that could be preventing some of the files from being backed up?)

Are you basing this 300GB on what the job says, or how much space is showing in the Media List for the tape?  (Compression settings could shrink your backup down in size)

Do you have DFSR running or does it have CIFS or NFS mounts on it?

Its is a windows client or a unix client?

You don't say what version of windows.

With DFS running - the files in the dfs directories do not get backed up when you just do the drive like J:

Open the BAR and look at your backup files.

For J it seems you will see some files - I guess that  these are NOT DFS-ed.

Now look in the Shadow Copy Compontes job drill down to DFSR

You should find your missing files.  (this is if you are backing up the SCC)

You should go to the tech site and look up all the docs about DFS and how to back it up.

Have a look at this TN:

http://www.symantec.com/docs/TECH148740