The "DBSS – Backup Media Free Space Is Too Low" rule only collects information from the root drive location.
How can this be changed?
The SQL Server cartridge collection itself retrieves only the drive letter using the Physical_Device_Name field which is stored as the backup location.
This is part of the out of the box collections in the database agent and cannot be changed.
A customization may be feasible using a User Defined Collection and customized rule based on this collection.