DUTs in the test lab are attached to a servo, and the servo
includes a USB stick that can be plugged in to the DUT
under various circumstances.  Although normally the USB
stick is unplugged prior to any test, there are error
conditions under which this can fail to happen.  See, for
example,  bug 754362 .

Some tests do the wrong thing in the presence of unexpected
removable storage devices, so it might be worth it to add a
DUT verifier that specifically checks that there are no
external block-storage devices present.

Advantages:
  * Bugs like b/64332561 will no longer be possible.
  * In cases where servo can be fixed, the failed verifier
    will trigger repair, and repair will include fixing the
    servo problem.

Disadvantages:
  * This adds a verifier for a condition that is both hard
    to explain or understand, and relatively rare.
  * In cases where servo can't be fixed, an otherwise working
    DUT will be excluded from testing, mostly unnecessarily.