At least by default, the is no volume named "Default" - there is just the pool "Default". Consequently ...
- If NW really requests "the media name Default", this can only occur when you start a recovery.
- If NW is requesting a media from the "Pool Default", this is during a backup situation.
Just go ahead and (re-)label a media to the pool.
Then, if you no not want to use this pool, look at your intended pool setup.
For a backup, NW would never ask for a specific media but only for a media from a certain pool.
Obviously you have mis-pooled. Let us know which pool you want to write to and the pool's criteria.