can't make bootable usb on mac

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http://askubuntu.com – I've been trying to do this since yesterday, following the instructions given on ubuntu.com's official page here: http://www.ubuntu.com/download/help/create-a-usb-stick-on-mac-osx I can complete every step without error. However, when restarting the mac, the flash drive simply reports 'missing operating system, press any key...'. From there, there's nothing I can do except a hard reset. I've tried Unetbootin, but that won't create a usb that's bootable from a mac. edit: I've also fixed the partition tables with rEFIt, but still no joy. Any suggestions? (HowTos)