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.
Cheers,
Tobias