Safely Remove ensures any writes sitting in a Windows buffer gets written to the drive. These writes could be very small, like an update to a file directory entry, but could be very critical to the data on the drive being usable. If Windows says the drive is not removable means something has a file or folder open on the drive. Be sure everything using the drive is closed. If unable to determine what is using the drive shutting down Windows works. And I don’t mean turning the system off, I mean shutting it down.