can't create a bootable Nbsd USB key, from linux, unetbootin or dd alike

[spermwhale_warrior]

Hello.
Though I think I may have been able to do it in the past, all my attempts fail now : I try to create a bootable usb key, which would contain the installer, AND on which I would install the system itself.
I try with unetbootin(I am in debian 9 now), while booting the unetbootin menu appears, but can't find any image to boot on.
And with 'dd if=image.iso of=/dev/sdc'(see, I took the whole disk, not the partition sdc1, maybe I was wrong ?), it isn't recognized at all by neither of my two computer.
We can find a way to create it, or to run the installer on a virtual box(never done that, or for a Windows more than 1 year ago), and install it on the key from the virtual box. I would add, that my computer CAN boot from a usb key, and I put all way to boot from it(fdd, hdd, cd, etc) one after the other before the hard disk, in the boot menu, to be sure I don't miss something.