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.