Trying to find out the best way to do this. I've tried several images (including the memstick-serial image) on a 4GB USB drive. None of them go past the boot process of the Soekris to show me any output on the serial port. Not sure why. Thought the serial image was geared to do that.
It is a known issue that the 6501 BIOS does not currently support booting off USB devices.
You will need to use a mSATA or SATA drive. You will need a way to either install the full version and enable the serial console OR load the nanobsd image on the drive.
For a mSATA drive, you will need a converter/ adapter to convert it to either SATA or USB for use on another computer. It can then be regarded as a fixed disk drive during the installation process.
For full install:
Place the mSATA (or SATA) drive into another computer and run the liveCD installation. Accept all the defaults and go into the WebGUI. Head over to System -> Advanced -> Check 'Enable serial console'.
Shutdown the system and move the mSata unit onto the Soekris mSata port (the adapter is not required here). For SATA drive, connect to the onboard SATA port instead.
Hook up your serial cable and reconfigure the interfaces and IP addresses over serial console on the first boot.
For NanoBSD installs:
Connect the mSata drive with adapter (or SATA drive) to another computer. Follow the installation guides for installing the Nanobsd image onto the drive. This typically just means running Physdiskwrite (for windows) or DD (for *nix OS) and targetting the mSata drive.
Move the drive to the Soekris and configure the interfaces and IP addresses over serial console as per the full install.