When you plugin your USB stick into a computer and copy some files from or to USB stick and then tried to eject it out by using Safely Remove Hardware. 

Although you have already finish copied files and close all the program and folder that related to the USB stick but you still will received a warning message prompt out, “The device ‘Generic volume’ cannot be stopped right now. Try stopping the device again later.” This problem come out is because some processes running in the background are accessing your USB drive while you cant just plug out the USB stick that will make a risk of corrupting the data inside it.

Until now, your only way seemed to be shutting down the computer. Not any more, for here comes a very nifty little utility. Read along to find out more.

EjectUSB is an extremely tiny (250 KB) utility that finds out what all applications are using the USB disk, kills them all mercilessly and releases your disk from the Windows jail, all within a matter of seconds.
All you have to do, is download the zip archieve, extract it to your USB disk and run the executable. Visit this link to watch a video of EjectUSB in action.

Note: Remember to close all unsaved files in USB drive before attempting these.