What is this disk geometry thing

From Soekris Info Wiki

What is this "disk geometry" thing?

If you burn a CF card on your PC, it will get its disk geometry values from your PC's BIOS. These are not necessarily the same values the Soekris comBIOS sees. If you are running a boot loader which uses absolute sector numbers (Lilo), you'll probably never notice this, as such a boot loader can handle the anomalous geometry. However, if you're running a boot loader which is FS-aware (most modern BLs), you might end up with a system that apparently refuses to boot.

The solution is to pass fdisk the geometry data (C/H/S values) as displayed in the serial console when you boot up the Soekris, when burning the CF. After that, things should run without a problem.