Remove Drive Error

I've been having an issue when I try to remove the drive after using portable apps.

When I try to remove the USB drive, I go to "remove device safely", but I get an error saying that I can't remove it and to try later. I also tried going to my computer, right clicking on the drive and "eject" but I get the same thing.

No applications are running, nothing from the USB drive or the internal drive. I went to windows task manager and it appeared that firefox was still running (under processes) but not applications. Once I ended the FF process, I was able to remove it.

What can be causing this?

Thanks.