See what files are being written to (like iotop but for files, not processes)
Solution 1:
lsof may help if you know the process you want to watch:
lsof -p <pid> -r 5
-r is the number of seconds between checks.
Solution 2:
You can use sysdig:
sudo apt-get install sysdig;
sysdig -c topfiles_bytes;
There's also an interactive (top-like) terminal program that might be useful in your case.
csysdig;