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'd been burning way too much time trying various ways of making a USB stick bootable, and couldn't get any of them to work, but when I tried UNetbootin it worked great the first time.