Hi Stefan,
As i understand you are unable to run a backup on any media and
you are asked to insert a overwritable media.
What you can do is:
1. Go to Tools -> options on properties list on the left, click
on Media management. Check for Media Overwrite portection level and Media Overwrite portection option.
In the protection level, set it to "Partial" and check the "prompt before overwritting imported media". In the option, select "Overwrite recyclable media first and then scratch media".
Then, Inventory the tape intended for use.
After making the mentioned settings try to run a job.
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