external USB bootable devices are not supported

Installer really install Arch Linux on A fixed harddisks. But when user choose some external USB devices it is making problems.
Grub always install its machine loadable code to The fixed harddisk or even to a loop partition. As a result it is not possible to boot from external bootable device. It is very interesting phenomena. Because fixed harddisks are really supported. External USB harddisks are very fast in todays times. So it would be A good idea to think about forcing Grub boot loader to install all of its parts correctly.
Sure, when user select fixed harddisk device, it is absolutely all right and system work. Even hybernation mode is correctly supported.