USB device activity when not mounted

I can understand why I would see activity on a USB device when it's first plugged in. But why do I see continued activity (i.e. the light blinks on a usb disk or memory stick)? When I umount one of these, they keep being beat up on and it makes me nervous... At what point is it sync'd and safe to unplug? I assume when the umount returns, but what's going on after that?

Gary
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