Can I boot from a USB disk?

yes, you can boot intel macs from USB devices. reboot and hold option at the chime. this will boot you into the startup manager. choose the external usb as the as the boot drive and proceed. of course this assumes that you've got linux already installed on that usb drive.

If I have a MacBook that won't boot (let's say the hard drive is corrupted) can I still boot from an external USB hard drive? I understand that holding down option gets you into the startup manager -- but is that still true with a corrupted drive? (ie, does the "startup manager" have anything to do with the software on the internal hard drive?)

See, obviously, I'm worried about buying a computer where I can't boot into target disk mode and fix a corrupt drive. Therefore, I'm wondering if I have problems, can I boot up from an external USB drive (like an iPod) that has the OS installed on it as well as diagnostic and tech tools so that I could then repair that computer...

Thanks for your help!

Simply... yes.

Slightly longer... the Startup Manager has nothing to do with the software on the internal HD so you should always be able to choose a boot drive by holding down option at startup, even if your internal HD is hosed.

Starting with Option pressed looks on each drive for a bootable system, so it knows which drives icons to display. A severely corrupted drive could make this process fail, and not allow you to boot from any drive. In that case, try booting while pressing Command-Option-Shift-Delete (the Delete key above Return). This tells it to ignore the normal boot drive.

Malcolm Rayfield wrote:
Starting with Option pressed looks on each drive for a bootable system, so it knows which drives icons to display. A severely corrupted drive could make this process fail, and not allow you to boot from any drive.

This should never happen because of a corrupted drive: the BootROM of the Mac is responsible for selecting which operating system to run from some partition. It is a part of the Mac's hardware, not something loaded from a hard drive or other storage media. (This is the main reason for its primitive user interface.)

The storage media supplies the boot loader code that actually loads the OS, so drive problems can prevent the OS from loading or its partition from showing up in the Startup Manager, but no problem on one drive should prevent other boot sources from displaying their icons if they contain an OS capable of booting that Mac.

What can affect the Startup Manager is corrupted PRAM or fundamental hardware problems, like power management issues. This is why resetting PRAM or the System Management Controller (SMC), & so on are recommended procedures for this type of problem.

Should you be curious about the technical details of the boot process, this Apple developer document is a good place to start.

Thanks everyone for all your help!