LTO Ultrium 4 append backup data
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We've just bought a new Dell Power vault for writing our backups off to tape.
we've got 5 tapes that are going to be cycled round on a monthly basis (1 tape holds 1 month of backups, giving us 5 months of backup) 

Because of the size of the data, we're planning to write to the tapes in weekly batches, however using Windows Server 2003 R2 NTBackup, the option to append to the tape is greyed out, we can only replace any existing data. This is going to cause us to big problems.

Can anyone bring some light as to why this option isn't available?
Is it maybe that we haven't written to this particular tape yet?
Is this a limitation of the tape?
Is this a limitation of NTBackup?
Could it be a driver problem maybe?

Thanks in advance,
Andrew