How do I unmount a key drive connected to the Xyboard?

[dooowopper]

I connected a key drive to my Xyboard tablet using a micro USB host cable. After I disconnected the key drive from the tablet, a message appeared on the tablet that the disconnect of the key drive was unexpected and that the drive should have been unmounted before disconnecting it.
How do I unmount a key drive connected to my tablet with a micro USB cable?
Thanks.
Richard