These days many linux distribution use hybrid ISO images.
Hybrid disc formats include the ability to be read by different devices, operating systems, or hardware. In the past, one example of this use was for a disc that supported both Microsoft Windows and Macintosh installations from a single disk image (by containing several file systems).
The problem when writing these iso images to an usb disk is that the usb flash drive needs to be recognized as harddisk. Hybrid iso’s add a MBR (Master Boot Record) in the first 512 bytes of the ISO image and pads the end of the image with zeros, so that the size of the final image is a multiple of 1M. Thus created MBR will have one partition listed, starting at offset zero (by default) and ending at the end of the hybrid image.
When booting a dell Poweredge with the auto option it recognizes the flash disk as a floppy disk, to prevent this the type of the flash disk has to be set to “Hard Disk” instead of auto.
Set the emulation type to “Harddisk” from “Auto” to make a hybrid iso image bootable from usb flash drive.
Also be sure to have the “USB Controller” set to type “On with BIOS Support”
And now have fun with your bootable usb FLASH drive and your poweredge!