Christoph Pfisterer - 2009-06-16

By and large rEFIt should already support this. It doesn't care if a disk is internal or external, so if it detects the disk at all, it should display all partitions and operating systems on it, and allow selection of a specific operating system by setting the active partition flag in the MBR.

There are several known problems with Apple's firmware and with the operating systems that may prevent you from booting from an external hard drive:
* Sometimes Apple's firmware just refuses to boot from an external drive. It is unknown what the exact conditions for this are.
* Newer firmware versions (~ starting mid-2008) don't wait for the CD drive to spin up during boot, causing rEFIt to miss the contents of the CD in the boot menu. This may affect external hard drives as well. The problem could be fixed in rEFIt with a workaround, but it requires major structural changes, so it hasn't happened yet. You can work around this by holding the Option key to bring up the built-in boot menu, then wait for the external drives to show up, then choose the hard disk that has rEFIt installed on it.
* BIOS wasn't really designed for booting from multiple hard disks, so there may be random issues with the boot loaders of the operating system you're trying to use.
* The operating system must support booting from USB/Firewire. If it's not careful, it may re-initialize the bus or its drivers, and cause its own boot drive to disappear "unexpectedly". At least plain Windows XP needs patches to prevent that.