MBLD 4TB- how many TBs that can actually be used?

My MBLD still shows 2.9/4TB in the control panel, but when I want to copy a 480GB folder over, it stated “you need additional 420GB space”. Does it mean I can use only 3GB actually? Or is it because of too much file fragments on the disk?

Thanks

You are probably connected via WebDAV.

http://community.wd.com/t5/My-Book-Live/WebDAV-Primer-Microsoft-vs-WD-My-Book-Live-etc/m-p/576975#U576975