What's required to make removal of a mounted USB stick safe?

[Ryan Stone]

Currently FreeBSD stands a fair chance at panicking if a mounted USB drive
is removed while I/O is in flight.  Does anybody know what work is involved
to have the kernel safely recover from this case?  Losing data from the
drive is expected of course but there's no reason that the entire kernel
has to crash.

A co-worker has been looking at this but I don't feel that we understand
the problem well enough to produce a real fix.  All that we've been doing
so far is papering over the explicit panics without having a full
understanding of what we're doing.