After adding new disk the free disk space and used disk space values is shown correctly.
However the total space value have not changed. Because of that the utilization value is wrong.
The total value is collected once during the Health Monitor setup as usually this value does not change.
It does not collected by the collecd service with other statistics.
After adding new hardware, execute the following command:
# /usr/local/psa/bin/sw-engine-pleskrun /usr/local/psa/admin/plib/scripts/setup-health-monitor.php