I'm trying to boot Xubuntu from USB Flash Drive on my old laptop Fujitsu-Siemens but it shows "Missing Operating system. Operating System not found". Although the Flash Drive works on my current laptop. Can I make DOS USB Flash Drive with (x)Ubuntu Live CD on it? I tried various windows programs to make a bootable USB, and Xubuntu Startup-disk without success.
This appears to be a problem in the system, not a problem in the creation of the Live Ubuntu.
For newer systems
Manual Boot from USB
If your system is a new one, manufacturers usually allows you to manually select your boot device by click a specific key during boot (mostly F8 or F12). When you click it, you should be able to select your USB Pendrive as a boot device.
If your USB that doesn't work enter your BIOS setup (This is also achievable by clicking a specific key, usually Del or F2), search for a setting like
Automatic Boot from USB
(This is not recommended if the previous step works, explanation at the end of this paragraph)
Now if you followed this step, make sure that you rearrange the boot order to have the USB after your hard disk drives when you are done. Because some systems, will not start if you have a "non-bootable" USB device connected (like a USB Mouse, or an empty USB flash drive, etc...) and will show you a black screen. It isn't dangerous or anything, but it can keep you confused for hours worrying that your system is bricked, and forgetting about this USB problem.
For older systems
Booting from USB
In old systems, there wasn't any option to boot from USB devices and none of the options above are available. If that was your case, consider this question:
Booting from CD
Booting from a CD can probably be easier than booting from a USB in older systems. You just need to do the same steps explained at the top, but instead of doing it for a USB, you need to do it for a CDROM drive.
Install Plop Boot Manager http://www.plop.at/en/bootmanager/download.html on usb1 flash stick and make bootable usb2 flash stick. Boot from usb1 (with plop) add the second usb2 stick, wait 10seconds and from the PLOP menu choose USB. After that you will boot from the second usb2 flash stick with no problem.
If you're curious the image is named