Panic when doing installworld/kernel with DESTDIR=USB memory

Hi

I have a repeatable bug. I get kernel panic when doing installworld/kernel
to a USB 3.0 memory on Macbook Air (USB 3.0 port I think). FreeBSD is
11.0-CURRENT from Apr 27th.

This can be avoided by mounting synchronized (mount -o sync).

Is this a known problem? If not I might be able to try get a core dump and
investigate further.

/Johannes

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