I've attached a WD Elements 2TB drive to my pcengines APU1 as external storage and just found that with the disk attached SeaBIOS crashes on boot. :/
The controller is USB2, not USB3.

Both the stock rom "Build 4/5/2014 (current production)" and a self-compiled coreboot rom with SeaBIOS master from git (rel-1.9.0-43-g76327b9) exhibit the same issue.

I'm not sure if http://www.seabios.org/pipermail/seabios/2015-November/009910.html is the same issue since it doesn't have log messages and doesn't seem to complain about it not booting at all.