Hi
There are two places where you would adjust the settings to get this working.
1. Set the Overwrite protection period and append period on the media set to ensure that the tape is appendable and overwritable, depending on the type of job you have fired. If in the backup job, you have set the job to overwrite, it would look for overwritable media, else it would look for appendable media and if it doesn't get appendable, it may switch to scratch media or recyclable media.
2. You need to verify whether the order of selection, is scratch first or recyclable first by going to tools-> options-> Media management. So, when an overwrite job runs, it would have two choices, a recylable(one whose OPP has expired) and scratch. Choice will depend on the setting you do here.
Understanding of these settings will help you design a backup strategy so that you dont get into this kind of trouble and also you dont lose data because of accidental overwrites.
For detailed information on Media management, consider following document:
http://support.veritas.com/docs/192265
Please mark it a solution, if that helps.
Thanks