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.

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 ...