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.