The tool you want is
lsof, which stands for list open files.
It has a lot of options, so check the man page, but if you want to see all open files under a directory:
lsof +D /path
That will recurse through the filesystem under
/path, so beware doing it on large directory trees.
Once you know which processes have files open, you can exit those apps, or kill them with the
Hope this helped you out.
-Until next time!