If you ever happened to attempt to eject your flash drive or external hard drive on Windows system, but failed with an error message of "the device 'generic volume' cannot be stopped right now. try stopping the device again later", then there's a process which still "holds" the file or the drive inside.
In Linux, you can use lsof command (short for List of Open File) to see which process holds the file and we can just use the kill command to terminate the process. The same situation is not the same in Windows where Task Manager doesn't give us any ability to see this kind of information.
In order to fix those error, you can use UnLocker or Process Explorer to find out who's process is still holding the drive and you can terminate it in order to make it releaseable from the system.