@dillstone:
We've got several of the netgate 2g and 4g nano installs. I haven't been able to get any of them to upgrade directly to 2.2 or 2.2.4 from 2.01 and 2.03. I've been having to take a spare firewall and configure it, and then swap or crack the case with a pre-imaged CF. The auto-upgrade gets stuck looking for the root device and I've poured over this and other forums trying to figure out why. I haven't tried slow-stepping up the upgrade like you mentioned.
Interesting, I have a spare unit too, I'll try the direct jump on this to 2.2.4 and see what happens, does yours allow you to revert back to 2.0.1 in case of a failed upgrade ?