How to install Windows 7 from a USB key or external hard drive? There are a few commands you need to master with the built-in command-line tool called ‘diskpart’. A step-by-step guide.

As I documented the process of preparing an external USB drive to install Mac OS X 10.6 Snow Leopard, it’s only right for me to do the same for its counterpart Windows 7. It turns out doing this for the latter is much simpler than for the former. It should only take a few minutes to prepare the disk, plus the time your computer needs to copy the contents of the Windows 7 install disc or image.

Installing any operating system from a flash or hard drive has the advantage of being faster, because optical discs can only spin so fast – or rather, so slow as the spindle allows.

Here’s how.

Ingredients

What you’ll need to get this super-spiffy install medium:

  • A Windows 7 RC or RTM/GA (Home Premium, Professional, Ultimate) install disc or image
  • A working Windows XP/Vista/7 computer to perform the preparations
  • A 4GB or bigger USB key or external hard drive

Preparing the disk

  1. Pop the external disk into a vacant USB-port.
  2. Open a Command Prompt window, either by hitting the Windows key in Vista/7 and typing cmd or selecting Run… from the start menu in Windows XP and typing cmd.
  3. Then, in the black box, type diskpart – if you’re using Vista or up you’ll be prompted with a UAC window to which you must consent. XP users will be brought to diskpart right away, unless you’re using a Limited Account (if so, switch to an Administrator privileged account or do a Run As…)
    Diskpart intro screen
  4. We need to find out which index the USB device is at, so we’re sure we’re preparing the correct disk (and not, say, your C: drive). Enter list disk. In my case, my USB key is located at position 1. Check which number yours it at before you copy-paste the following commands!
    DiskPart: List Disk operation
  5. To tell DiskPart we want to start preparing disk 1, enter select disk 1.
  6. Type clean.
  7. Now we’re going to repartition the drive into one primary partition, so type create partition primary.
  8. To make the partition we just created the active one, simply enter active into the box.
  9. Then, we’ll want to format the active partition as FAT32. This is done by typing and entering format fs=fat32 quick. The quick keyword will wipe the file table instead of wiping the disk sector by sector, which takes a lot longer. If you’re concerned with the wiped data being completely gone, omit ‘quick’.
  10. Finally, type assign so the drive gets a letter in Windows Explorer for easy access.

Copying the files over

You now have a clean, ready to be copied to, disk. What follows is extremely simple:

  1. Insert your Windows 7 installer disc into the tray or mount the installer image with a tool like DaemonTools.
  2. Open the disc’s/image’s contents by browsing to it using Explorer.
  3. Select all files and folders (CTRL+A or ‘Organize > Select All’).
  4. Copy the files using the right-click menu or hitting CTRL+C.
  5. Paste everything onto the root of the prepared USB drive.

Your USB drive is now bootable

Restart your computer and make sure you boot from USB. The installation procedure will continue as it would from a disc, only faster. Happy installing.