Kurt Jefferson wants El Capitan to let his drives go!
Sometimes the drive won’t unmount no matter what, and I get the dreaded “The disk wasn’t ejected because one or more programs may be using it” or “The volume cannot be ejected because it’s currently in use” messages.
He’s had this happen routinely with a Mac mini using flash drives and external hard disk drives (HDDs). I’ve definitely seen some of this behavior, though it’s not routine. If OS X “understands” why it can’t unmount a drive, starting a few releases ago, it will tell you. But the dialogs Kurt is seeing aren’t the informative kind that tell you which app is involved.
Read more here:: What to do when a hard drive won’t unmount in OS X El Capitan