Umounting defugalties

I used to have all the usual newbie problems with not being able to unmount a CD or flash memory, but finally learned not to try it while some session is looking at the device, and definitely not while something is using it.

If it did refuse to umount, then I could usually find the proper process to kill. Didn't happen often enough to worry about.

Now, I am using several usb drives for real, off and on all day and need to find the proper process for determining what is "busy". I have been playing with the command.

As a control case, if I mount a drive and use it then make sure that all windows and consoles looking at that drive are closed, then the drive usually umounts ok. But if I leave just one view of that device and umount, then it is busy forever, even if I close everything and exit Xwindows.

So what is the proper procedure for determining what is hanging up? Not an essay, just a man page name will do. I remember an item with a Linux command for that purpose, but my memory will not access it. Googling with umount in the search item gets about a thousand years of reading, especially on a phoneline locked down to 26k (you broadbanders don't know how lucky you are).

Thanks dorno