More Mac 101, our ongoing series of tips and tidbits for new Mac users.
Update: As the comments point out and Low-End Mac confirms, support for USB booting was present in the Mac OS 9 era with the introduction of dual-channel USB in 1999, available first on the slot-loading iMac and one model of the AGP PowerMac G4; however, your mileage and performance may vary when trying to get these machines to boot Mac OS X from USB devices (as has been previously reported on TUAW). Our apologies for the error!
When I wrote my post Tuesday about booting a Mac off an external USB hard drive, I was surprised how many people followed up to tell me that it wouldn't work, and that I needed a Firewire drive to externally boot a Mac.
While [much] older machines did indeed need Firewire for bootable external media, that is no longer true. Since the release of the Intel Mac computers [and well beforehand -- see update above], and with Mac OS X 10.4.5 or later, you can start up from an installed system on a USB hard disk. Here's the Apple support document that tells you how to do it.
I didn't think it was possible either, and when I bought my MacBook Pro last year, I was wishing I could boot from a USB drive because of the easy availability of inexpensive storage. A little online research revealed the obscure truth. USB booting generally works fine from a bus-powered portable drive as well as a powered USB desktop drive. In my case, I booted up just fine from a portable with no external power supply. Check the support document linked above for more details.
So have at it -- either install a clean version of Leopard or Tiger on the drive, or use your favorite bootable-backup utility to clone your existing install to the USB volume. It's a good thing to know as Apple seems to be determined to bury Firewire on the lower priced laptops it sells.
It is also possible to boot an Intel Mac from a USB flash drive. That is a bit more involved, and there are several methods. One is Das Boot, a free utility from Sub Rosa that allows you to convert original disks from DiskWarrior, Drive Genius, TechTool Pro and others to a flash drive. Let Google be your friend on this. Many people have put their favorite rescue utility on a bootable flash drive to save them from any problems in the field.
Before you ask; no, USB does not support target disk mode -- a sore point for MacBook Air and unibody MacBook owners.
Thanks to Dave and others who wrote in asking about this, and thanks to many readers who wondered about the ability of older PPC Macs to do this as well.