You can use Htop.
Set it up to show RBYTES,WBYTES,and IO RATE.
Have it sort on IO RATE.
Now move this to desktop 2.
Now when your extreme IO starts, click on desktop 2 to show Htop.
Even if response seems non-existant, there usually is a response even though there is a long delay. So it will probably switch to desktop 2 while the activity is still happening. If it does, the culprit will be at the top of the list.
If things are so slow that you can not switch to desktop 2, just go to it as soon as you can and sort on RBYTES or WBYTES. If your mystory process is really doing extreme IO, it ought to show as one of the largest IO processes.