usb3 zpool disappears

[R. Riehle]

I have an amd64 machine running freebsd 8-stable with three zpools, two 
on internal drives and one on an external Western Digital usb3 drive.  When 
I rebuilt my world last week the pool on the external drive refused to mount.  
The other two pools are fine.  The drive and the usb3 card seem to be 
intact.  

Some failed attempts to solve the problem:

1.  zfs mount -a
No response but doesn't hang.

2.  zpool mount bigpool
hangs

3.  zpool status
hangs forever.  I waited for 24 hours.  No instability.  No logs.  No 
appreciable cpu usage.  It just stays there.

4.  zfs status
See #3

5.  zpool mount -f bigpool
hangs indefinitely just like #2, 3

Snippets from dmesg:

:
xhci0: <XHCI (generic) USB 3.0 controller> mem 
0xfeaf0000-0xfeafffff,0xfeaee000-0xfeaeffff irq 17 at device 0.0 on pci4
xhci0: [ITHREAD]
xhci0: 64 byte context size.
:

ugen4.2: <Seagate> at usbus4
umass0: <Seagate FreeAgent GoFlex, class 0/0, rev 3.00/1.00, addr 1> on 
usbus4
umass0:  SCSI over Bulk-Only; quirks = 0x4100
umass0:2:0:-1: Attached to scbus2
da0 at umass-sim0 bus 0 scbus2 target 0 lun 0
da0: <Seagate FreeAgent GoFlex 214> Fixed Direct Access SCSI-0 device 
da0: 400.000MB/s transfers
da0: 953869MB (1953525167 512 byte sectors: 255H 63S/T 121601C)

~ $ uname -a
FreeBSD xxxxxxx 8.3-STABLE FreeBSD 8.3-STABLE #0: Fri Jun 22 17:08:30 CDT 
2012     xxxxxx at xxxxxxx:/usr/obj/usr/src/sys/CUSTOM  amd64

google shows others have had the same problem but I haven't found any 
solutions.

Any insights???

Robertsen A. Riehle