Unable to boot on USB key on Lenovo Thinkpad T440

[Kata Goto]

Hello all,

I recently (this week), recieved a Lenovo Thinkpad T440 and I want to
install FreeBSD, so I have disabled the secure boot and force the legacy
boot mode only.
Then I have downloaded an USB image and write it via
    $ dd if=FreeBSD-9.2-RELEASE-amd64-memstick.img of=/dev/da0 bs=64k

(I also tried with 10.0-RELEASE)
And my laptop refuses to boot on it, even if I force to boot on it (via
<F12>).
My USB key works well on other computers (they boot on it) and my laptop
boots well on an Ubuntu USB key.
I have also tried to boot on an external CD Drive, but nothing change :/

I am running out of ideas.
If you have the same issue or new ideas, please, let me know.

In advance,
Thanks for your help.