Hello,
The reason of this behavior is the fact that Parallels Plesk Panel (PP) is not counting the disk usage at the external storage. You can verify it at the PP database with the help of the following query:
mysql> select dom_id,dbases from disk_usage limit 3;
+--------+--------+
| dom_id | dbases |
+--------+--------+
| 1 | 6144 |
| 2 | 4096 |
| 3 | 0 |
+--------+--------+
3 rows in set (0.01 sec)
Where dom_id is the id of the domain and dbases is the space that is occupied by the databases.
Thank you