Please check for any hung process.
One similar issue occured with us.
Our SFV-490 server "/ file system" got full.
We checked for /var directory and any other large file, but we could't find anything.
Only /proc directory was showing 1.5T in size.we checked that directory and found the size of the directory of PID of "fmd" process was having the size 58 GB. Then we just restarted the "fmd" process and then our / file system was back to normal condition realeasing a total space of 40 GB.
Cheers.