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 think that one of the most useful developments of the past couple of years has been bootable USB sticks. Not just "LiveUSB" sticks, from which you can actually run Linux, although those are ...