We have rules determining how overwritable media is selected once a job needs a pirce of overwritable media. The rules determine the selection order and the exact order can be affacted by settings within Backup Exec.
What Jaydeep has not explained is that every set of rules always contains overwrite recyclable media in any media set which can cause media to move between media sets.
The reason this rule is always somewhere in the order is that the product has been designed to always try and complete a backup if one is scheuled and not fail it just because there is not enough media available in a given media set.
These rules and the settings that affect the order are covered in our admin guides
The reason partitions lock down the control is that instead of controlling the content of the media set, they control the device the job uses and then we split up the library into a number of virtual devices (the partitions) that only contain certain slots. This results in less or even no movement between media sets (depending on how you set it up you slot partitioning) but more likelihood that a job will be queued waiting for overwritable media when you check your overnight backups
