Tripo

Hi, I have a strange problem... I have an external hard drive (with power) connected by USB to the laptop with Windows 7, when manually disconnecting the hard drive from Windows, the antivirus (last version) starts scanning it and Windows pulls out a window saying that the hard disk is in use and can't be disconnected. I have the option of automatic scanning to removable drives (and I wish I did not have to remove it ) What can I do ? And why could this be happening ? Many thanks.