I don't know if there is some kind of "effective" limit for these drives...
300 GB is too much. It is not system drive, so no swap file there. And i could fill any other drive to the last byte. Although this is my first WD red.
...if you have any services or Applications running on the DNS-320 that may be trying to reserve HDD space, reducing the effective storage.
How can i check what services or apps are running?
I sometimes run p2p app. Most of the time it is disabled. I was also wondering about that but i couldn't find any setting that could be the problem. (maybe i dont know where to look) I also sometimes run ftp server. Again, none of the settings seems to be the problem. I have 3 users, one for PC, one for my smartphone when i am in house WIFI, and one for smartfone when i am out of house WIFI network. Only setting that looks to me that could be the problem are quotas for users, but i never set any of them and they are disabled. All aps came with NAS, and i didnt install any 3rd party aps. I didnt even try other apps.