But I could not get any solution for this specific problem
Start by backing up the NAS, as the data is at risk.
Do you have SMART snapshots enabled on the NAS? If you do, then I suggest
determining a reasonable retention date (I use three months myself, but you could go longer).
delete all snapshots in all the shares that are older than the retention date
wait a bit and see if free space improves on its own
If it doesn't (or if you don't use snapshots), then try running a balance. On current firmware you can run that directly from the settings wheel on the volume tab - not sure if you can do that with 6.2.4 or not. If it's not there, you'll need to schedule the balance using the maintenence schedule you'll see on the volume settings wheel.
After this is sorted, I suggest upgrading to current firmware.
If you are using SMART snapshots, the problem will eventually recur - because monthly snapshots are retained forever. So after you upgrade firmware, switch to Custom snapshots and explicitly set retention. I also suggest setting them so they only make snapshorts on changes. Note Custom snapshots weren't available in 6.2.4, so you do need to upgrade the firmware in order to do this.
