I do not prefer CD / DVD for OS installation as its slow & time consuming.
From past few years I am using HDD / Network installation for CentOS.
One thing I like about live media is it can be used on any computer & better option to rescue when Windows crashed.
To create USB boot download latest version of unetbootin from http://unetbootin.sourceforge.net/
For CentOS / Fedora you need to install p7zip
On Ubuntu you can install using following command.
sudo apt-get install unetbootin
Run Unetbootin, select Ubuntu iso image & select appropriate usb drive.
Once USB is created, boot your computer using USB Boot.
Unetbootin can also be used to create usb from other live media, such as Fedora.
You can find latest list of support OS from http://unetbootin.sourceforge.net/
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