XFCE? Sometimes it's thumbler that's working on pictures or movies. In many cases it comes not to an end. Sometimes it's xine or mplayer that still stay after click on the cross right-up.
Try the command given by michaaa62 to find it out.
I can confirm that this happens to me, too. I also use the XFCE spin.
I copied some .avi files to a usb stick and couldn't unmount after it was done for a while. I received the same message (output) about the device being busy. Yet, there was no activity AFAIK or could see from the usb flash drive.
Edit: The file or file(s) appeared to be there in entirety as well. Or so it appeared.
I also waited a while afterwards and it did ultimately unmount. I don't recall the time period but 15 or so minutes sounds about right.
'sync?'
"You might try to use 'lsof /dev/YOUR_USB' to look for applications, that prevent the unmount of the device."
Thanks. That looks interesting, too.
I'll try that next time.