I've had this problem with two separate external USB drives (my Time Machine backup drive and a different one), and also my USB Flash Drive. Every time I try to eject any of these drives, I get this dialog box: This happens every single time with every external drive I've tried, even if I mounted the drive and then immediately attempted to eject it without using it in any way. If I hit "Force Eject," it claims that it has been ejected, but it still remains mounted and gives me the error dialog box when I remove it that I have removed a drive without ejecting it. I haven't been able to correctly eject a single drive since the OS upgrade. Does anyone have any ideas about how to figure out what's causing this and how to get it to stop?