Found it in the documentation:
If you restore individual items from an Information Store backup set that is on a device other than a backup-to-disk folder, then Backup Exec must temporarily stage the entire database to a path on the media server to extract individual items. Because of the potentially large file sizes that are created in the staging location path, system volumes should not be used as a staging location.
To enter the path, on the Restore Job Properties pane, under Settings, click Advanced, and then enter a path in Path on media server for staging temporary restore data when restoring individual items from tape.
It's at the bottom of the screen in Advanced during the restore setup.
Vince