When booting off a USB key, using Lubuntu, and Ubuntu, i get this message when booting
"Unable to find a medium containing a live file system:"

Can someone help me? Thanks!

Some more information would be useful.

What exact steps take place during boot before that message appears?

What make/model computer?

Does the BIOS include an option to boot from USB?

How did you set up the USB key for booting?

Have you tested the USB key in a computer that you know is capable of booting from USB to verify that the device is bootable?

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It boots into everything, but just before the login screen

Gigabyte 990FXA-UD3

Yes, i switched the order so it boots to that first

Yes

You didn't answer a very important question that Frank asked:

So, how did you set your USB key up to boot? It's real important that you pay attention to your hardware and its capabilities when trying to make bootable USB devices.

I used PowerISO. I made sure i had the right hardware too. (Amd 64bit)

Huh? "It boots into everything, but just before the login screen"

No, it boos up. But just before the login screen it says that.

If the computer has an optical drive, I'd try burning a boot CD/DVD from the *.iso and booting from that for testing purposes.

I did a websearch for "Gigabyte 990FXA-UD3." It seems to be a chipset designed to receive the actual CPU. I have no idea what that means; perhaps it would be useful to consult the manual for that motherboard.

If it works correctly except for that error then I would leave it.

Normally it won't boot past that point/error.

When Linux distro's started following Knoppix and building live cd's they used some tricks to get it to work. I assume some leftover issue while making this usb has caused it to leave that error. I don't use live creators myself too much anymore. I just install it to a usb just as if it were a real internal hard drive. It doesn't leave any of the live part (or shouldn't.)

By the sounds of it you are not booting from the USB stick at all. How did you set up the BIOS to boot from USB? You should be booting the machine with the USB stick in, then setting BIOS to boot from hard drive first and changing the order of hard drives so that the USB stick is the first to boot. If that isn't working then it's likely that the method you used to create the bootable USB stick didn't work properly.

I don't think this is a BIOS problem. BIOS would complain that there's no operating system, it would not make a complaint about not finding a "live file system". You have an operating system attempting to load and it is that which is complaining. Perhaps the USB you formatted was made into a boot disk but is missing the file system. I call that the root file system; however since it is intended to be a LiveUSB boot, they've coined the phrase "live file system"; it's the same though. Linux needs the kernel, and a root file system to run. I'd recommend you retry formatting that USB.