Just for fun, I tried this too before. Worked fine, no problem. I saw all disk access flying by, pretty spiffy. Then I closed the window and did some other stuff. Later, I decided I wanted to see it again. No dice. Got the already in use error message.
So I had a look at top, and there it was, still in use. After I killed it, it worked fine again. But as soon as I closed it, and tried again, it refused to work.
That means that this fs_usage process runs rampant, even after closing the terminal and all. So, if you run into this, do the following:
- Open terminal, type 'top'
- look out for a process called 'fs_usage', and note yourself the PID of it
- press q (to quit top)
- enter 'sudo kill PID' where PID is the number you noted before
- now everything is fine again, run fs_usage by typing 'sudo fs_usage'
HTH with your problem.