I’ve had trouble getting minicom’s xmodem to fully upload a Soekris firmware image over USB serial adapters, and I’ve never been a big fan of minicom in the first place. I like good old Kermit. Recently I discovered how to get Kermit to speak xmodem, and used it to update the firmware on my Soekris Net4801.
First, install kermit and lrzsz. Next, connect your Soekris box. Now fire up kermit and go!