I'm a switcher, and I remember looking up how to boot from USB if the bios supports it, and e-mailing somebody to ask why their shareware program didn't support booting from a flash drive.
They said flash drives can't handle quite as many read/write cycles as a hard drive, so it's not recommended for them to go through the hammering a boot sequence gives the drive as it creates and deletes files all over the place. (I have no idea how this translates to the Mac, but it seems to be common-sense that large modern operating systems do A Lot Of Stuff before you see your login, so I'd expect many tiny files to be created and deleted in XP and Tiger).
Apple Ibook G4 12.1 Laptop
This suggests that booting from a flash drive all the time (for security or portability reasons) would not be good, because after a matter of weeks or months it might give up the ghost.
Ibook G4 Operating System Download
It occurs to me that this might mean a flash drive is a really good choice for a minimal system 'rescue disk' in the way people used to keep a boot floppy somewhere. This is because a flash drive seems to be noticeably more robust than floppy, zip, cd-r or hard drive, as far as keeping it lying about and expecting it to work is concerned. I haven't actually tried this myself (I tend to clone the main disc to the firewire disc and hope it all works), but it's an idea.
Apple Ibook G4
(Sorry if everyone already knows all this, but it's probably worth pointing out in case anyone's just thinking about it and falls foul of this)
Explore the world of Mac. Check out MacBook Pro, iMac Pro, MacBook Air, iMac, and more. Visit the Apple site to learn, buy, and get support. . Macs up through the Powerbook G4 1.33 GHz, and at least some desktops up to and including the same vintage, and maybe a few later pre-Intel PPC Macs (not sure why not all the pre-Intel PPC Macs), will boot SOME USB drives from USB 1.1 and 2.0 ports/drives into OS 10.4.x (at least 10.4.6 and above-I didn't try earlier versions)-NOT OS 10.3. Any iBook G4 can install using this as t he first one in late 2003 came with Mac OS X 10.3 and stopped support at Mac OS X 10.4.11 while the latest iBook G4 came with Mac OS X 10.4.2 and stopped at Mac OS X 10.5.8, so ultimately, all iBook G4 models were upgraded to Mac OS X 10.4 Tiger at some point. For more information, see our Apple Hardware Guide listings for the 12-inch 1.2GHz iBook G4, 14-inch 1.33GHz iBook G4 with Combo drive, and the 14-inch 1.33GHz iBook G4 with SuperDrive models.