nsurlsessiond is not only iCloud Drive. If you block it with Little Snitch , you won't be able to browse some websites or submit any forms. I ended up to block the ausyd-edge.icloud-content.com (17.248.155.9) only which is the only one responsible for the massive data usage. I'm on 10.10.1, have never had a problem like that before. I have no photos on cloud drive, only text files, there is no reason for it to download whatever it is downloading continuously.