Can't boot 8.0-BETA4 from USB stick

Greetings all,

I have an amd64 system (Asus A8V-MX) which boots 7.2 from a USB flash
drive. Trying an 8-STABLE kernel from Saturday (identifies as
8.0-BETA4), the system can't boot - here is the last bit of the kernel
messages (typed, with comments):

[witness performance blah blah blah]
root mount waiting for: usbus2 usbus1 usbus0
uhub0: 2 ports with 2 removable, self powered
uhub1: 2 ports with 2 removable, self powered
root mount waiting for: usbus2
uhub2: 2 ports with 2 removable, self powered
root mount waiting for: usbus2
ugen2.2: <TOSHIBA> at usbus2
umass0: <Toshiba TransMemory, class 0/0, rev 2.00/1.00, addr 2> on usbus2
umass0:  SCSI over Bulk-Only; quirks = 0x0000
root mount waiting for: usbus2
umass0:0:0:-1: Attached to scbus0
Trying to mount root from ufs:/dev/da0s1a
ROOT MOUNT ERROR:
If you have invalid mount options, reboot, and da0 at umass-sim0 bus 0
target 0 lun 0  <---- This is suspicious
da0: <TOSHIBA TransMemory PMAP> Removable Direct Access SCSI-0 device
da0: 40.000MB/s transfers
da0: 3875MB (7936000 512 byte sectors: 255H 63S/T 493C)
first try the following from  <---- The rest of the message that was
cut off above
the loader prompt:

etc. etc. until the mountroot> prompt, and after that even specifying
the device doesn't work (and ? doesn't list it). It seems to my
non-hacker eyes that the device is not attaching until after it should
be.

Any help would be appreciated, and I can provide further information as needed.

Thanks,
James Butler