In the old days we all had CD-drives, but most modern computers these days don't have an optical drive. This makes things even more efficient, because you need to think smart. Some people choose to have an external optical drive, but why all the hassle when you can have a bootable USB or a bootable partition on your harddrive. There ar many guides on the internet, how to get that done manually. Again, why the hasse when we have UNetbootin, the perfect software for booting your favorite ISO's from your harddrive! UNetbootin works for Linux, Windows, and Mac.
REMARK! Your bios bootup settings should be UEFI only or UEFI first. The legacy thing creates an syslinux error.
Think about how many CDs and DVDs we don't need to throw out anymore! Perfect efficiency!
You might get into trouble if you buy a new laptop with Windows 8.1 installed, because the bootloader is not fond of the UNetbootin. What I did, was format a partition on an external usb-drive as FAT32 and make it bootable and install the iso there. Then you need to go into the boot options on your laptop when booting. This will enable a boot into your iso. In my case Kubuntu 15.10.
If your unetbootin is unable to find your device, you can force it with:
unetbootin installtype=USB targetdrive=/dev/sdbX In my case sdb3 for my external HD.
Additional notes: http://www.everydaylinuxuser.com/2015/11/how-to-install-ubuntu-linux-al…
Update 2021-06-26
As of today the repository for unetbootin for ubuntu 21.04 does not work. Grab the binaries directly from https://unetbootin.github.io/ and install dependency "mtools". On my machine unetbootin failed to detect partitions on the disks. The best tool is "Gnome" "disks". In this tool you can select the partition and use the button "additional partition options" to restore partition image. This works great with a Kubuntu 21.04 iso file.