- Download rEFIt-0.14.dmg from SourceForge, install, and reboot.
- Download the linux image, in my case Ubuntu 12.04.03 for 64-bit Mac (AMD64) desktop CD.
- Open a terminal window and convert the linux image by typing
hdiutil convert -format UDRW -o converted.img.dmg [original].iso. (In my case this step seemed to be completely unnecessary because the converted file was identical to the original when I checked its MD5 hash.) Check the size of the disk image using:
du -h converted.img.dmg
- Insert a flash drive 1 GB or larger into a USB port on the Mac. Open Disk Utility from /Applications/Utilities and select the flash disk. Make absolutely sure that you have selected the flash disk and not your hard drive! Erase the disk specifying Mac OS Extended (Journaled) format.
- Select the Partition tab, hit the Options button, and choose GUID Partition Table. Create a partition large enough for the Ubuntu disk image using Mac OS Extended (Journaled) format.
- From the command line, find which disk is your flash drive by typing: diskutil list
- Unmount the disk using: diskutil unmountDisk /dev/diskN
where N is the disk number you found in the previous step.
- Copy the image over to the flash drive:
sudo dd if=converted.img.dmg of=/dev/diskN bs=1m
- Restart the Mac with the flash drive still inserted and select the external drive icon ("Boot Legacy OS from HD") from the rEFIt menu.
- While Ubuntu is starting to boot press Escape then F6 and select the noapic and nomodeset menu options.
Booting Linux Live USB on a MacBook Air
I have a MacBook Air with an Intel i7 core running OS X Lion 10.7.5. Booting linux from a flash drive was not straightforward, to the point that when I finally got it working I laughed with surprise. Evidently doing such things is not part of the one true Apple way. The instructions were cobbled together from from here and here with a fair bit of trial and error. Needless to say, the procedures documented on the Ubuntu website did not work for me.