How to upgrade to or clean install Mac OS X 10.6 Snow Leopard from an external USB or FireWire drive? It’s easier than it sounds. A step by step guide.

Optical discs are so 2000! Whether you got the disc at retail or acquired it through dubious sources like The Pirate Bay (which I don’t condone), you can re-partition an old iPod or external hard drive and use it to install OS X. Installing an OS from USB/FireWire is remarkably faster than its disc-based variant. It’s also not so prone to scratches and dirt, a problem many disc owners face daily.

Required Ingredients

To get this speedy install medium baked and ready, you’ll need:

  • Mac OS X 10.6 Snow Leopard install DVD or ISO
  • External USB/FireWire drive or old iPod
  • A working Mac computer to partition the above drive

Convert OS X 10.6 Install DVD to DMG

If you purchased the Snow Leopard disc – like most – you may want to convert it to a disk image for back-up purposes. This step is not required, so you may skip over it if you want.

  1. Launch “Disk Utility”
  2. Select the Snow Leopard DVD in the side pane
  3. Select “New Image” from the top
    diskutil1
  4. Give the DMG a name to save it somewhere
  5. Click OK

Partition external drive

There are a few things you need to pay attention to for your Mac to be able to boot from your external drive. You can achieve this by – once more – using the Disk Utility described above.

  1. If not already open, launch “Disk Utility”
  2. Select the external drive you want to use, not one of the underlying partitions
  3. Select the “Partition”-tab
  4. If you still want to use the disk for other storage purposes, you should create 2 partitions. The first one will be around 10GB and will allow for installing Mac OS X 10.6.
    diskutil2
  5. Select the first partition (in my case called “BOOT”) and click the “Options…“-button.
  6. Select “GUID Partition Table“. This is the only type of partition table that Intel Macs can boot from. If you format the other partition as FAT it will still be usable on Windows or Linux.
  7. Make sure that the first partition is formatted as “Mac OS Extended (Journaled)”.
  8. Then, still having the first partition selected, click the “Restore”-tab at the top.
    diskutil3
  9. Go select your install image (or select the physical DVD) and drag the destination partition from the left pane to the destination text field.
  10. Click restore.

Once ready, make sure you have backups handy (if you use Time Machine you’re set to go), plug in your disk. You can then start the installation from the desktop or reboot and hold down the Option/Alt key at startup to select your drive.

Installation will work as it would when running from the DVD. Only faster. If you created a DMG, you’ll also have a backup in case you lose the install disc (or it gets broken).