How often do mounts and umounts fail because you can't determine what is using the device? How many Times can you not eject a CD because something's got it open? How many times have you experienced a "file browser" that keeps a directory open even after you've navigated out of that directory and clicked the "refresh" button? Who's got this thing open??? Use thefuserprogram to find out:
[slitt@mydesk slitt]$ /sbin/fuser -mu /d/d: 1693(slitt) 1891c(slitt) 1894 1894c(slitt) 1907 1907c(slitt) 1908 1908c(slitt) 1909 1909c(slitt) 1910 1910c(slitt) 1912 1912c(slitt) 1913 1913c(slitt)[slitt@mydesk slitt]$
You get the owner and the process ID. From there you can research usingps ax.
Be sure to look at the fuser man page for details on other info you can obtain. You can even kill all processes accessing a file, though I highly recommend against such a heavy handed move.