usb related panic in 6.1-prerelease

[Surer Dink]

All,

I have two USB hard drives attached through an "Adaptec" USB2 card. 
The disks were working fine (although seemed a bit slow ~3MB/s random
reads/writes as reported by iostat while rsync was copying from one to
the other a complete freebsd install).  The drives were left connected
to the machine unmounted.  At some point the following appeared on the
console:

umass1: Phase Error, residue = 32
(da2:umass-sim1:1:0:0): AutoSense Failed
Opened disk da2 -> 5
umass1: Invalid CSW: tag 2575 should be 2576

however I did not notice this message until later connecting to the
machine through ssh and trying to mount the _other_ disk (the disk on
da1).  the kernel panic'ed; through remote kvm I was able to copy down
the following (which was below the CSW message):

Fatal Trap 18: integer divide fault while in kernel mode
...
current process = (...) mount
...

the instruction pointer was at 0xc081daab, which is:

0xc081da9f <__qdivrem+47>:      mov    $0x1,%edi
0xc081daa4 <__qdivrem+52>:      mov    %edi,%eax
0xc081daa6 <__qdivrem+54>:      mov    $0x0,%edx
0xc081daab <__qdivrem+59>:      div    %ecx
0xc081daad <__qdivrem+61>:      mov    %eax,0xffffffbc(%ebp)
0xc081dab0 <__qdivrem+64>:      mov    %eax,0xffffffc0(%ebp)
0xc081dab3 <__qdivrem+67>:      cmpl   $0x0,0x18(%ebp)

(probably) relevant system details:

6.1-PRERELEASE FreeBSD 6.1-PRERELEASE #0: Sat Mar 11 19:09:49 EST 2006
/usr/obj/usr/src/sys/GENERIC  i386

usb1: OHCI version 1.0
usb1: <NEC uPD 9210 USB controller> on ohci1
usb1: USB revision 1.0
uhub1: NEC OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub1: 3 ports with 3 removable, self powered
ohci2: <NEC uPD 9210 USB controller> mem 0xfebfe000-0xfebfefff irq 23
at device 1.1 on pci2
ohci2: [GIANT-LOCKED]
usb2: OHCI version 1.0
usb2: <NEC uPD 9210 USB controller> on ohci2
usb2: USB revision 1.0
uhub2: NEC OHCI root hub, class 9/0, rev 1.00/1.00, addr 1
uhub2: 2 ports with 2 removable, self powered
ehci0: <NEC uPD 720100 USB 2.0 controller> mem 0xfebffc00-0xfebffcff
irq 16 at device 1.2 on pci2
ehci0: [GIANT-LOCKED]
usb3: EHCI version 1.0
usb3: companion controllers, 3 ports each: usb1 usb2
usb3: <NEC uPD 720100 USB 2.0 controller> on ehci0
usb3: USB revision 2.0
uhub3: NEC EHCI root hub, class 9/0, rev 2.00/1.00, addr 1
uhub3: 5 ports with 5 removable, self powered
umass0: vendor 0x0402 USB 2.0 Storage Device, rev 2.00/1.03, addr 2
umass1: vendor 0x0402 USB 2.0 Storage Device, rev 2.00/1.03, addr 3

please advise.