Shutdown -r under -current hangs on RPi3

From time to time, but seemingly more often lately, a Pi3
get stuck during shutdown -r. The machine is running -current
from a mechanical usb hard disk through a powered hub. No micro
SD card is used. Once up it's quite stable.

The console reports

login: Sep 23 08:20:37 pelorus shutdown[224]: reboot by bob:
Stopping sshd.
Waiting for PIDS: 1063.
Stopping cron.
Waiting for PIDS: 1073.
Stopping powerd.
Waiting for PIDS: 1002.
Stopping devd.
Waiting for PIDS: 752.
Writing entropy file: .
Writing early boot entropy file: .
.
Terminated
Sep 23 08:20:43 pelorus syslogd: exiting on signal 15
Waiting (max 60 seconds) for system process `vnlru' to stop... done

Waiting (max 60 seconds) for system process `syncer' to stop... Syncing disks, vnodes remaining... 4 0 0 0 done
All buffers synced.
Uptime: 23h31m45s
Khelp module "ertt" can't unload until its refcount drops from 1 to 0.
Resetting system ...

At that point all activity ceases. The only clue I can recognize is that
the red power LED remains off, as if FreeBSD never relinquishes control
to the Pi firmware, which turns the LED back on at powerup. Power-cycling
results in a normal reboot.

Thanks for reading, and any suggestions for further testing.

bob prohaska