one option for this is to use unlocker,
you can removed your external drive even it is in used
just right click unlocker then set to unlock.
it will tell you what function is still running from it why it's not removable,
then when you unlock it, your screen may be blink.
and you can removed it.
but be careful to use unlocker
don't use unlocker to delete the thing you don't understand
it can damage your system