Endian Firewall boots from my daily backup USB stick / from my UMTS modem USB stick !?!

UMTS USB modems mostly are bundled with an USB storage, which contains the Microsoft Windows Drivers.
USB storage sticks mostly have a boot manager installed on their master boot record (MBR), which jumps in when Endian Firewall boots. The boot manager then needs manual interaction or does not find the real boot image at all and remains stuck.

You need to remove the boot manager from the MBR in order to let Endian Firewall boot correctly.

With linux (or the Endian Firewall itself) you can achieve this using the following command:

dd if=/dev/zero of=/dev/sdX count=512

where the X in sdX needs to be the letter which identifies the drive. For example, use /dev/sda if the USB storage is the first, /dev/sdb if it is the second SCSI type device in your system.