Comment 6 for bug 363958

The problem (at least in 12.04) is that udev is called in /scripts/init-top and dropbear is called right away in /scripts/init-premount/dropbear. There's no time for udev to load the network module and the card to be probed and recognized before dropbear tries to initialize the network.

A quick & dirty solution is to add a "sleep 10" inside /scripts/init-premout/dropbear, right after the "Starting dropbear" line. A not so quick and dirty solution is to create another script that adds the sleep after udev. The proper solution is to add logic to udev and make it wait until all the modules have been loaded. This has do be done by canonical or upstream (don't really know who.)

Note that this bug only happens when the FRAMEBUFFER variable is set to "n" (my case, since plymouth makes it really hard to unlock the drive remotely and I can't afford that just for a cute graphical screen.)

BTW, I'm surprised that a simple problem like this hasn't been fixed since 2010, which is the date of the last post.