No serial number

No serial number semi-brick

If you have an old machine (e.g. a B2) without a serial number and you upgrade it to build 542, and you will probably get something like this:

could not activate this XO
Serial number: SHF00000000

or like this when you try to activate:

No serial number in mfg data
No serial number
Use power button to power off

Then you need to use the following workaround.

Step 1: USB boot

Take the USB key you used to upgrade your machine and mount it on another system. Edit the file /boot/olpc.fth to look like this:

\ OLPC boot script
null$ to ramdisk
boot n:\boot\vmlinuz root=mtd0 rootfstype=jffs2 console=tty0

Note that the first line starts with "backslash space"

Now power down your laptop, insert your USB key, and power up. The laptop should boot normally.

Step 2: making it permanent

Get into the development console using Alt=, and open the terminal. Use su to get a root prompt. Then cd to /boot and make a backup copy of olpc.fth. Then start vi on olpc.fth

su
cd /boot
cp olpc.fth olpc.fth.saved
vi olpc.fth

Edit the file as in the olpc.fth given above, then save and exit

Now your laptop should boot normally

Credits

Many thanks to Mitch Bradley for walking me through this.