Bootloader bombing out with USB disk attached - suggestions?

[Mason Loring Bliss]

Hi, all. I noticed this this morning, on rebooting:

    http://imgur.com/IX2vIgw

I've got a USB hard drive plugged in, and when it's plugged in, FreeBSD won't
boot, giving me the error message I photographed and linked, above. If I
unplug the USB drive, FreeBSD will boot. I can then plug the drive back in
and use it.

I'm writing because I'm curious about how I'd go about debugging this
usefully.

Here's an excerpt from what dmesg says about the device:

da0: <TOSHIBA External USB 3.0 0> Fixed Direct Access SCSI-6 device 
da0: 40.000MB/s transfers
da0: 953869MB (1953525168 512 byte sectors: 255H 63S/T 121601C)
da0: quirks=0x2<NO_6_BYTE>

-- 
 Mason Loring Bliss         mason at blisses.org        http://blisses.org/  
"I am a brother of jackals, and a companion of ostriches."  (Job 30 : 29)