Hi
Please create a media set, by going to media tab. Set Overwrite protection period to "n-1', where n is the number of days, after which you would like to bring the same tape back and overwrite it. So, if you want to bring back the tape of first week on 3rd week, set Overwrite protection period to 13. Also, from your description, i can see that you dont want to run an append job, so set append period to zero. Then create a backup job and submit it to the library, by selecting library in devices tab and the media set created above in the media tab.. I do not see a requirement of partition in your case. As, when you submit the job to the library, it will protect the tape anyways, so you dont need separate partitioning.
Mark it a solution if this helps.