I believe dossrv (which implements the fat filesystem) is synchronous,
so you shouldn't have to do anything special before removing the
stick.  What are the errors you get the next time you try to mount the
stick?

> you have to restart usbd.  usbd doesn't handle device ejection as
> far as i can tell.

I'm pretty sure that it's more nuanced than that.  I've been able to
eject and re-insert usb devices on uhci interfaces.  I think the
problems with usbd not noticing device additions and removals is
limited to ohci interfaces, and I'm not sure why yet.