Single yellow characters on Mac when Ubuntu is booting

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http://askubuntu.com – I have burned a Ubuntu disk and am booting from it on my Mac. However, it keeps going to a black screen with a single yellow character on a burgundy background. Usually an S and a weird ñ. When booting it always displays a little icon in the bottom of the screen which looks like a keyboard, followed by an arrow, followed by a person in a Circe. Sometimes the install/check disk/etc menu comes up before the weird yellow symbol, sometimes it doesn't at all. When the symbol is up, no amount of pressing buttons or anything works. Is this a Mac hardware problem? Is my disk bad? Thanks, Sam (HowTos)