1)USB/ESATA CD-ROM/DVD-ROM --- This is the traditional one and I believe that there is no need to mention anymore
2) Bootable USB thumb CD-ROM/DVD-ROM drive (pendrive) --- This method requires the user to have an intensive knowledge of mass production of USB thumb drive.
3) Bootable USB/ESATA portal virtual ROM ZM-VE200 (i-odd 2501) --- This devices give you a very way to make any ISO images to be bootable either ROM, HDD, and both ROM/HDD. Please note that you have to use ROM mode ONLY in order to make the device to be absolute CD-ROM/DVE-ROM mode to install software.