launchd with high disk usage launchd is producing a lot of I/O activity. In the last day and a half it managed to write 12 GB of data while reading 500MB.

I the logs I do not see anything suspicious. Any suggestion at where could I look?
Try to use the lsof utility

sudo lsof -p 1
# or 
sudo lsof -p $(ps -U root | grep -w "/sbin/launchd" | head -1 | awk '{print $1}')


EDIT
There are can be other user launchd processes running with different process IDs, run

ps aux | grep sbin/launchd


To check PIDs of other instances of launchd.

It should list opened files by launchd. Try that to find there something suspicious.
You can also try an app called fseventer, it shows read/write events to the filesystem, or use app like Grand Perspective or Disk Inventory X to find this newly created huge file and investigate further what it is.