USB, NetBSD 6.1.5/amd64: freezes when 2 umass connected

[tlaronde%polynum.com@localhost]

Hello,

On an NetBSD 6.1.5/amd64, when I connect a second USB connected disk to
the machine, NetBSD freezes. Unable to connect remotely; hard reboot
required.

Questions:

1) The machine has two usb ports, with uhub0 and uhub1 first attached
resp. to these ones; the uhub2 cascading from uhub0 and uhub3 from
uhub1.
uhub2 has 6 ports removable;
uhub3 has 8 ports removable;
Since in /dev/ there are only 8 devices (from usb0 to usb7) could this
be the problem? (6 + 8 = 14, even if I have only one USB device---first
disk---and the second disk is only the second device; but how are the
device nodes assigned to one USB port?)

2) The two USB disks are from the same vendor (Western Digital) but not
exactly the same model (not the same capacity). Could the USB driver be
confused by two similar devices connected to the same(?) USB tree?

3) Physically, on the machine, there are USB ports on the rear, and USB
ports on the front. Does somebody know if front ports could be
"duplicating" rear ports, that is slots on the front be in fact
connected to the same ports as the rear ones causing conflict?

I'm trying to find what is causing this misbehavior. And a freeze is
rather annoying for a node that is mainly supposed to be administrated
from remote...

TIA,
-- 
        Thierry Laronde <tlaronde +AT+ polynum +dot+ com>
                     http://www.kergis.com/
                     http://www.arts-po.fr/
Key fingerprint = 0FF7 E906 FBAF FE95 FD89  250D 52B1 AE95 6006 F40C