USB-boot flash memory stick - Fixit mode do not start

[Alexender]

I try both 8.0-RELEASE and 8.1-RC official memstick images. I try write them to memory stick by that commands:
# dd if=memstick.img of=/dev/da0 bs=10240
# dd if=memstick.img of=/dev/da0 bs=512
I also try to write image from Windows by win32diskimager-RELEASE-0.2-r23-win32

I think all was writen well in all cases - in /dev appear /dev/da0a - i try to mount it - all was fine. I try to boot from it - all was fine too - appear sysinstall.

But when I try to go to Fixit mode and choose USB - sysinstall tell me that there is no USB-device. I try that on 3 different PC - all the same. What I did wrong?