Once, I had a difficulty to put my Pre in USB Update Mode. The usual way to turn off the device then turn it on again by holding down volume up button and power button didn't work. So I search the internet for any clue, and I found it here.

These what you need to do:
a. Connect the USB cable to the AC charger, then into a working outlet
b. Turn off your webOS device
c. Remove and reinsert the battery
d. Press and hold volume up button, and connect your USB cable (which already connected to AC charger and working outlet) to the device

Note: You can press and hold R key as an alternative to volume up button

e. Release the volume up button (or R key),  when the screen showing large USB icon







f. run webOS Doctor

I've tried this and it works. Hope this works for you, as well :)