Can't boot freebsd from usb device - Invalid slice

[WaW]

Hello.
I'm trying to install freebsd 6.2-STABLE to usb device. After
installation done successfully and reboot i getting messages like
this:

Invalid slice
Invalid slice
No /boot/loader

FreeBSD/i386 boot
Default: 0:ad(0,a)/boot/kernel/kernel
boot:

typing "da(0,a)/boot/loader" and many other variants had no success.
Also i tried to install manually, allocating full disk for bsdlabel
slice - same result.

My motherboard is asus a8n-sli premium, and it is capable of booting
from usb. Recently i successfully installed and booted opensuse from
the same usb device.

Is there any solution to make freebsd boot from usb device?

PS as usb device i mean my usb flash and ata disk with external case,
i tried both of them.

-- 
Vladimir K. aka WaW