Cloned live-only USB boot drive
When you clone an Ubuntu iso file to a USB drive, the USB drive will be a bootable live drive, that can be run to 'Try Ubuntu' or to install Ubuntu into an internal drive.
dd is a cloning tool, when used as described in the original question.
There are other cloning tools that are safer, for example 'Disks' alias
gnome-disks, the 'Ubuntu Startup Disk Creator' in Ubuntu 16.04 LTS and newer versions, 'mkusb' and 'Win32 Disk Imager'.
USB drives that are cloned from iso files have the ISO9660 file system which is read-only. This is a feature of the system, that can be used on CD/DVD drives and it is inherited when cloned to USB drives.
There are other tools, that extract the files from an Ubuntu iso file to a file system in a USB drive. 'Unetbootin' and 'Rufus' are extracting tools. 'mkusb' is an extracting tool, when it is creating a persistent live drive.
Persistent live USB boot drive
If you want to use the pendrive both to boot Ubuntu live and to store files, I suggest that you create a persistent live drive.
Restore to a standard storage device
If/when you no longer need need the drive for booting Ubuntu live or installing Ubuntu, you can restore the USB drive to a standard storage device with 'mkusb-dus'.
See the following links and links from them,
A simple 'Do it yourself' method
New: A simple 'Do it yourself' method is described in the following links.
If you only want to use the pendrive to run Ubuntu live or install Ubuntu, a cloned system is the best alternative. If you want to make a persistent live drive I think mkusb provides the best alternative.
But if you want to use the pendrive to run Ubuntu live or install Ubuntu and use the remaining drive space to store data or transfer data between computers, the 'Do it yourself' method is a good alternative. It is also a good alternative, if you want to understand the details by doing it manually.
It is enough to
- Create an MSDOS partition table and a partition with a FAT32 file system and a boot flag.
- Extract the content from the iso file to the mounted FAT32 file system to create a drive (live drive or install drive) that boots in UEFI mode.
You must add a bootloader separately, if you want the drive to boot also in BIOS mode. There is a compressed image file, that helps you fix this, in cases where it is very difficult to install the bootloader for BIOS mode.
There is a general description in this first link,
There are detailed instructions in this second link,