Plan B: All you have to do is turn off your mac (unplug it as well), and eject the disk manually. Then, reboot and it should be good to go. I honestly don't have a clue about how macs work internally, so I'm not sure how you would prevent it from occurring in the future. Try reinstalling the drivers for it maybe?
Ummm sometime mac has more trouble than a pc. I happen to use both but with a mac this problem could be caused by a virus or by a simple computer glitch I hope this was helpful.
OP: Sometimes my drives don't eject, and i usually find it's because I've been watching a film from it and I haven't closed the application I played it with (like quicktime for example). If I quit that then eject it usually works.
If you're having more problems try logging out or logging back in, or restarting. Sometimes Finder is still accessing the drive even though you're not doing anything yourself. You just need to wait a bit for it to finish what it does, sometimes it acts up and won't eject so restarting usually solves that.