> > I got the impression that I would be able to UEFI boot from multiple > different devices, eg a USB drive with UEFI as well as the hard disk. >
FYI, after rebooting with the USB drive in; # efibootmgr BootCurrent: 0000 Timeout: 1 seconds BootOrder: 0000,0004,0001,0003,0002 Boot0000* Gentoo <- this is the UEFI booted OS Boot0001* Hard Drive <- this the same drive as Boot0000, but IIRC fails to boot Boot0002* Network Card Boot0003* USB HDD <- this is the same device as Boot0004 Boot0004* UEFI: Patriot Memory PMAP, Partition 1
