In my own experience when I have something open, for example, in open office, then open an open-office document from my usb card, even after I close that document from the USB card, I cannot eject the device until open
office is closed entirely. This has only happened a few time for me
in linux, but happens all the time in MS-office in Windows. There,
even after closing every office instance, I sometimes have to go into
the taks manager and manually kill the office process...