If a USB drive is plugged into the system before power on/boot up then that drive can not be "Safely Removed" as it will always say that it is in use, even if you dont even view the contents. Ive tried it with a few flash drives and a USB3 hard drive. If you plug them in after the computer boots up then its fine. Im assuming its some kind of startup scan that remains active monitoring the drives, preventing safe removal. This was tested on a fresh install of Windows 7 x64 and the newest Nod32 v4.