amd64 shutdown during USB rsync

[Mark Carroll <mtbc%ixod.org@localhost>]

Hello. I'm trying out NetBSD 8.0 (GENERIC) #0 on an old Intel Broadwell
NUC. I have a couple of USB3 portable HDDs plugged into it, each with an
encrypted FFS filesystem mounted via cgd. Doing an rsync from one to the
other it gets around 800GB through but can't finish it off: each time it
does a little more then abruptly shuts the machine down, leaving nothing
interesting ending the logs. I've not seen this machine shut down
without warning in any other circumstance. Watching CPU temperature with
envstat -i 10 shows nothing interesting there: they seem to be down at
40C or so just before the shutdown. In case it helps,

# usbdevs /dev/usb0
addr 0: xHCI Root Hub, vendor 8086
 addr 1: Expansion, Seagate
 addr 2: Elements 25A2, Western Digital
addr 0: xHCI Root Hub, vendor 8086

Is there anything obvious I should be looking at or trying? After the
latest fsck I might see if there are any USB BIOS settings to tweak.

-- Mark