strange problem with external USB disks

I'm having a very strange problem with external USB disks in
HEAD.  I see the problem with a kernel from July 12th and also
with one from today.

I never saw it while I was running 12-STABLE (switched to HEAD on
July 12th).

Just for background I'm using a B350M-A mobo from Asus which
supports only USB3 (XHCI).

Also note that all external disks are in USB3 enclsures or docking
stations.

I can turn on an external disk and it is recognized and I can
mount it and use it for hours.

But, as soon as I turn it off, the port to which it is connected
effectively stops working.  The worst thing is that my keyboard
also dies, no matter whether it's a USB keyboard or an old one
plugged into the PS/2 port.  Strangely enough, the mouse is not
affected, even when it's plugged into the USB hub built into the
USB keyboard.

I state that the port stops working because turning the disk back
on or replugging the USB keyboard into the same port results in
no trace output from the kernel.

Replugging in the keyboard or exiting X still leaves me with a
dead keyboard and a working mouse.

Whether this is a problem with XHCI or CAM is unclear.  The fact
that the keyboard is also affected makes things really weird.

Since I'm running a custom kernel I'll try running GENERIC to
see what happens and report back.

-- 
Gary Jennejohn