Yeah, i had a friend with the same thing... got a external network hard drive... filled it 3/4 full.
Enabled the cloud stuff on it, not aware that it would try to SYNC... and had exactly as described above.
Thats often where alot of UPLOAD usage can come from.. enabling some form of cloud service, which is now UPLOADING all your stuff up to the cloud.
(and then, usually consecttively DOWNLOADING to another device now that has syncing enabled.)