Mine refuses to be "ejected" and gives me the same error. As Jack says, often a second attempt is successful, without doing anything else in between. Sometimes, I open an application, quit out of it, and then eject the flash drive. Since I use Excel most of the time before using the flash drive, I had assumed it had something to do with the Evil Empire.
I've also learned to be patient with the flash drive. When I first got it, and Eject didn't appear to do anything, I clicked Eject again. (I usually right click and select Eject) Now, I wait about 10 seconds, and when the icon disappears, I wait another ten seconds before I pull the flash drive out.