Hello,
It is strongly recommended that careful consideration should be made before setting the media overwrite protection level to None. When the overwrite protection level is set to None, all media is subject to being overwritten regardless of media set, overwrite protection period, append period, or any other property.
You may refer to the below technote.
(title)The basics of Advanced Device and Media Management for Backup Exec for Windows Servers
http://support.veritas.com/docs/192265
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