Why can't I set the space alert threshold for my volume?
The volume might be a legacy volume, which means it was created before QTS 4.0. The behavior and status of legacy volumes are not consistent with new volume types. Also, legacy volumes do not support advanced features such as snapshots and individual low space alerts.
To ensure that a legacy volume has enough free space, go to Control Panel > System > Hardware > General, enable the setting “Enables the light signal alert when the free storage size is less than the value”, and then specify a warning threshold. This setting issues a warning when total free space on the NAS falls below the specified threshold.