Disclaimer: Creating a bootable USB without using the downloaded ISO but the pre-installed Ubuntu is a little complicated for new users. If you want that, see the answer below. This answer gives two simpler alternatives.
Make a standard bootable Ubuntu USB from within pre-installed Ubuntu (simple)
Frist, check the installed version of Ubuntu by opeining a terminal (press Ctrl+Alt+T) and type lsb_release -a.
Note down the version number and OS Type (32bit or 64bit).
Second, download the ISO from http://www.ubuntu.com/download/desktop, by selecting the appropriate version.
Third, insert an USB drive (4GB or greater for Ubuntu, 2GB or greater for Lubuntu, Xubuntu).
The Startup Disk Creater takes two inputs, the location of the ISO file and the location of the USB drive.
Fourth, point the Startup Disk Creater to the ISO and the USB drive.
See the help on Create a usb stick on ubuntu for a simple set of instructions.
Also see a more detailed instructions on USB stick creation
Remastersys does not exist anymore.
The next section is obsolete.
Make a custom bootable backup Ubuntu USB from pre-installed Ubuntu (Complicated)
First, Add Remastersys source information to Software Center. Open Software Center and click on “Edit” and select “Software Sources.”
Select the “Other Software” tab at the top and you need to click the “Add” button, and copy and paste this:
deb http://www.remastersys.com/ubuntu precise main
And then click “+Add Source”, then click "Close" and completely close out of Software Center.
Add the PGP key for the Remastersys repository, in Terminal (press Ctrl+Alt+T) copy the two lines below one line at a time and paste (press Ctrl+Shift+V) in Terminal:
wget -O - http://www.remastersys.com/ubuntu/remastersys.gpg.key | sudo apt-key add -
sudo apt-get update
Second, install Remastersys. Open Software Center and search for and install remastersys.
Third, run Remastersys. Click on "Backup" and wait a long time till it finishes creating the ISO file.
Fourth, create startup USB following instructions above.
More detailed information and some qualifications on how to do this is here.
Hope this helps.
It should be enough to just cat the device. If your USB is, for example, /dev/sdb, you can do:
cat /dev/sdb > foo.img
to restore the image to a USB disk, do:
cat foo.img > /dev/sdb
Note that this approach will create an image file as large as the drive itself, even if the drive is almost empty. If the drive is not 100% full and you only want an iso of the used space, use genisoimage (install it with apt install genisoimage) instead:
genisoimage -o foo.iso /path/to/usb/mount/point
Best Answer
In order to make an Ubuntu live USB for installing Ubuntu in Windows, use Rufus. Rufus is the recommended application at the official Ubuntu website for making a bootable live Ubuntu USB on Windows. The USB flash drive you use should be 2GB or larger and formatted to FAT32. For Ubuntu 18.04 and later the USB flash drive should be 4GB or larger. The formatting procedure will delete all of the files that are already on the flash drive.
The instructions for preparing an Ubuntu live USB are found at the official Ubuntu How to create a bootable USB stick... webpage (see the step-by-step instructions below).
Open Rufus and select your USB stick in the Device dropdown menu.
Click the CD-ROM icon next to the FreeDOS dropdown menu, then find your downloaded Ubuntu ISO and click Open and then Start.
Click Yes when it asks to download Syslinux software.
Click OK to write in ISO Image mode.
Confirm that your USB stick is selected and then click OK to continue. It is very important to verify that the device that you are installing the Ubuntu live USB to is indeed your flash drive, so that you don't overwrite any of your system or personal files which may make your operating system unbootable.
When it is finished, just restart your computer and start using Ubuntu, or you can install Ubuntu.
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The Ubuntu live USB flash drive that you make this way will be bootable from a PC. In order to enable your computer to boot from the USB flash drive, you must enter your BIOS menu by pressing one of these keys: Delete, F2 or F10 as soon as the motherboard splash screen appears when the computer is booting. In the BIOS menu, you need to change the boot order so that the USB flash drive, which is usually called USB-HDD in newer computers, is the first entry in the boot sequence, and then save your new BIOS settings and exit the BIOS setup.