I've had issues booting some Intel Macs from USB thumb drives. Specifically, the Lion thumb drive that I made a few months ago worked fine with an early 2008 MacBook Pro, but didn't show up in the ...
The CDROM image was a firmware upgrade image from Dell (based on Linux). Mounting the image with iDRAC, it worked nicely. As one of the Servers had a dying iDRAC (Dell R320), I needed an USB stick.
The firmware included with the Raspberry Pi 2 Model B version 1.2 introduced the possibility of booting from a USB device: before that, it was only possible to boot from an SD or microSD card. But ...