dd if=/tmp/install-amd64-minimal-YYYYMMDD.iso of=/dev/sdX bs=8192k && sync

This will wipe everything on the USB key. Double check that there is nothing on there you want before proceeding. 
Make sure you have the correct device path! Note also that we need to target the device itself, and not a partition within it,
 so for /dev/sdX in the above command, use e.g. /dev/sdb and not/dev/sdb1; /dev/sdc and not/dev/sdc1, etc.


ref: http://wiki.gentoo.org/wiki/EFI_Gentoo_End_to_End_Install/Creating_and_Booting_the_Minimal-Install_Image_on_USB