Sometimes when you use an external hard disk and later opt to safely remove it,  you might receive a message saying ‘Cannot safely remove drive-because it is  being used by something else’ even though you may not be actually using the  drive any more.

This problem happens because, apparently, the focus of Windows Explorer is still  on the external disk and so considers it unsafe to remove the disk. A workaround  to this problem is to open a file in your local drive. For instance, you could  navigate to My Documents and open something in there. This means that  Windows Explorer has moved from looking at the external disk to accessing a  folder on the internal disk. This then usually lets Safely Remove Hardware release  the USB drive. If that doesn’t work, carry on using the PC for several minutes  without accessing the external drive and you should be able to Safely Remove  Hardware.

If nothing works, then just shut down the PC and unplug your device.