Can you install OS X on a flash drive or thumb drive?

Yes! You can install Panther or Tiger on an USB key, also called a flash drive or a thumb drive. This is handy for running Repair Permissions or Disk Warrior on an internal hard drive without using Target mode, or when cloning bootable hard drives with Carbon Copy Cloner rather than needing a 2nd external hard drive (you need to be booted from a drive that un-involved in the cloning process to reliably clone OS X drives).

Subrosasoft makes a utility called DasBoot which installs a bootable OS on thumb drives, but only if they are Firewire. What follows is the Do-It-Yourself approach, using USB. Standard disclaimers apply. If you break your Mac, you get to keep both pieces.

The process of installing OS X on a thumb drive is not simple (for non-techies) but it's detailed here. AND it's only possible because now you can buy these thumb drives that only 1 or 2 gigabytes. In the past, the largest thumb drive for sale was 512 megabytes, and that's just not enough space to install OS X. The article describes squeezing OS X on to a 1 GB drive but with only a few dozen megabytes of free space left at the end. A 2 GB drive or even larger is going to be more practical, especially when you start adding Utilities to the thumb drive.

In the Finder, try doing a File, Get Info... (‡I) on the folders System, Library, and Applications on your hard drive. Add up the amount of space they use... the total is probably at least 10 to 20 gigabytes. So in order to fit OS X on a thumbdrive, it has to be crammed in there, and sacrifices will have to be made to make room.

Here's the basic procedure:

Format the thumb drive with Disk Utility. Use either the Erase tab or the Partition tab and set the drive to 1 partition. Format it MacOS X Extended (Journaled). Turn off the OS9 drivers to save space. You CAN boot OS9 (on OS9 bootable Macs) from USB thumbdrives with no hackery involved. But to squeeze OS X onto a thumbdrive, you won't have room for 9 or classic.
You have to enable permissions on the thumbdrive with the terminal command: sudo /usr/sbin/vsdbutil -a /Volume/yourThumbDrive
Manually install OS X to the thumbdrive with Pacifist. Put the install disc in and click "Open Apple Install Packages" – select the install disk from the list and skip disc 2 when asked.
Choose 2 packages, BaseSystem and Essentials, from EssentialSystemSoftware, and Install Files to Other Disk... and choose your thumbdrive.
Delete the Chinese and Japanese fonts from /System/Library/Fonts
Copy /System/Library/CoreServices/SetupAssistant from your hard drive to System/Library/CoreServices/ on the thumb drive.
Copy BaseSystem.pkg and Essentials.pkg from /Library/Receipts on your hard drive to /Library/Receipts on the thumb drive.
Make your thumb drive bootable with the bless command: sudo bless –verbose –folder “/Volumes/myThumbDrive/System/Library/CoreServices” –bootinfo
sudo bless –verbose –folder “/Volumes/myThumbDrive/System/Library/CoreServices” –bootinfo –bootefi
This allows the Mac to "find" a bootable copy of OS X on the thumb drive.
Repair Permissions on the thumb drive with Disk Utility
For intel mac, you are DONE.
For PPC, boot into open firmware with ‡+option+O+F and type devalias as the prompt. You should see a ud: device corresponding to the thumbdrive. You can tell by rebooting into OpenFirmware with the thumb drive attached and detached and compare the output of devalias. The line that only appears with the thumb drive attached is the OpenFirmware address of the thumb drive. To boot the thumb drive from OF:
setenv boot-device ud:5,\\:tbxi
PPC OS X just doesn't support USB booting like Intel does. The terminal command nvram boot-device ud:5,\\\\:tbxi will set the thumb drive as the Startup Disk from OS X, but the Startup Disk system preference panel won't work to choose the thumb drive. Only on Intel, I'm afraid.

It's that easy! And by easy, I mean 'laborious'.

Also, after you test the thumb drive for bootability, run Keychain First Aid. If it complains about missing files, write down their names and copy them over from your hard drive.

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