Windows can't stop your 'Generic volume' device because a program is still using it. Close any programs that might be using the device, and then try again later
Solution:
1. Right click on my computer, select manage
2. Click on disk management tab
3. Left tab (which has probably "disk 1" labelled )on external storage drive right click and select "offline"
4. Now from task bar, from usb icon, click and select your device and remove it, Now it says "safely remove"
NOTE: when you plugin that external storage again, you must have to put it online by following above steps.
NOTE: when you plugin that external storage again, you must have to put it online by following above steps.