Ok, i’m back with the response from WD suport:
Thank you for contacting Western Digital EMEA Customer Service and Network Support. My name is[Deleted]
From the dashboard of the unit, it is true, you can only see the total capacity of the unit.
On the shares tab, you can see the usage space of each share on each volume.
On the Storage tab, under RAID, you can see the capacity of each volume.
A truer indication is to mount a share on a particular volume to a connected OS, such as Windows 8.1 or Mac OS X, on each disk, and then check the properties. This will give the usage.
The only way to check to see exactly what has been used, is to either generate the logs, and then check the report for df.hi in the zip or connect via ssh to the unit and actually run the command, df -h, which will give you the real time usage for each share.
I would advise you to report your recommendations / ideas of improvement in the WD Community forum. Our development
team reviews these posts on a regular basis, to see the most commonly requested features by our customers.
WD Community Forum: http://community.wdc.com/
WD Community Forum - Ideas Page: http://community.wdc.com/t5/New-to-WD-Community/Where-Do-I-Post-My-Ideas/td-p/122260
I believe that it is mandatory on a storage device to be able to monitor every disk.
So please vote for this here: http://community.wd.com/t5/Network-Product-Ideas/Diskspace-info-on-JBOD/idi-p/880179