I have Centos 6.3 installed on an external Western Digital drive with Firewire and USB interfaces. I want to be able to boot an iMac (2008, 3.06GHz Core 2 Duo) from this disk. The iMac has Mac OS X 10.5.8 and also a Window XP installation. I have tried holding 'T' on bootup for target disk mode but the external disk is ignored (presumably as it's not a Mac OSX image). I created an rEFit boot DVD which when booted in CD mode (holding 'C' on startup) displays three options, Mac OS (on internal drive), Linux and Windows. Selecting the Linux option unfortunately boots the Mac into XP. Three options are only displayed when the external disk is plugged into the Firewire port. If the external disk is plugged into a USB port the Linux option is not displayed and I can only boot into Mac OS X or Windows. This external disk will happily boot a Lenovo T410 laptop into Centos. My questions are: 1) Is it actually possible to boot into Centos on an iMac with an external hard drive. If so how do I achieve this? 2) Why is rEFit apparently booting from the wrong partition?