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I have a reformatted Mac mini containing only a fresh install of Ubuntu 9.0.4. The Apple boot loader as been removed, so I can't hold down option, or the c key to get it to reboot form the install DVD. I do have an external drive (with both a usb and firewire interface available) with a bootable cloned OS X system on it.

What steps do I need to go through to in order get grub to see the external drive, and to boot from it so that I can reformat the internal disk as an OS X drive?

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Take a look at this, it might help:

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This is not a GRUB problem, you have to setup your computer to boot from the usb hard drive. On a PC you usually hit one function key when you turn on the computer and it loads a boot menu where you select the device to boot from. Check if you have something similar on the Mac.

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grub is quite capable of booting from a specific device. the process you describe gets the BIOS to choose a preferred boot device, but once grub is loaded it can chainload a bootloader from any other device that grub detects. – quack quixote Apr 14 '10 at 2:05

I don't know if this will work but try rebooting and holding down shift-option-command and Delete. That used to force the mac to boot from something other that the internal hard drive. I haven't tried it lately.

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