I plan on calling a plumber, but in the meantime curiosity is killing me. My laundry is upstairs. Currently I installed a new washer. Everything has worked fine except last night I noticed some water damage along where the wall meets the ceiling, directly under my washer. There isn't any leaking under or around the washer. I have a standard drain in the wall (shown in picture one) which seems to be working fine. Also, I have a horizontal pipe running out of the wall and emptying into a floor drain. Upon closer inspection, this floor drain was backed up with water. I've never seen water dripping from the pipe into the drain, nor have I ever seen water in the pipe. I've ran the washer, flushed every toilet upstairs, ran every sink upstairs, and every bath tub upstairs and nothing effects neither the backed up water in the drain or the pipe coming out of the wall into the drain. Also, I've tried running a coat hanger and it seems to just get stuck at what looks like a 90 degree turn in the drain pipe about 6-8" down. Also, I measured the spot where the leak in the ceiling is and it's exactly where this drain is. My questions: what's the purpose of this drain? I know a lot of upstairs washrooms have drains in case of an emergency, but what's the pipe for? Is the clogged drain likely the culprit of my ceiling leak? Thanks and any help would be appreciated.

http://i.imgur.com/D2IdG.jpg

http://i.imgur.com/AG30o.jpg