I'm doing some rearranging of backup drives and erasing those I don't need. The quickness of El Capitan's "Erase" indicated it wasn't really doing much for security other than wiping the top level directory - so I tried secure erase (no more option for what levels - 1pass, 7pass). Unfortunately it's taking FOREVER, and DU will not allow us to Quit or Stop or Cancel the process - and will not Quit the program either. So all we're left with is to wait hours or even days for it to finish with no way to escape*. Unless someone here might know? Of course *Activity Monitor could Force Quit - or maybe a restart - but the lack of any straightforward UI bail out in DU is totally bogus. :-( Next time we'll have to do this on an idle computer and forget it for a day or so - per each drive.