Safely Remove Hardware query

[Neil F]

Often when I have finished with using a USB I click on the icon "Safely Remove hardware".
The USB  was being used only. within Windows Explorer to copy or update file(s). I exit Windows Explorer for both my main drive and the USB drive but I still get "Device is still in use" messages.
Since I am not on the Internet nor any (known to me) other program besides Windows Explorer I am puzzled as to why I get this "device still in use" message.

Comments please
Thanks

[king1]

often it is the antivirus software scanning the drive - best bet is shutdown then remove if you really must, otherwise just wait