Installing Windows from a USB drive is old hat to many, both consumers and professionals, but booting to Windows from an external drive would be a new trick.  Windows 8 has been designed with this type of usage in mind, which is unsurprising considering how much talk there is about the cloud.  A proper implementation of this would mean that low cost computers, shipped without a hard drive, could be readily sold.  Both Kingston and Super Talent have designed USB 3.0 devices which will have "Windows to Go" on them; fully able to boot to a full installation of Win8 on Intel powered machines.  Unfortunately there is a problem with WinPE installations on ARM based devices, as that method requires a wired network connection which may mean ARM devices would have to be sold with a USB to ethernet dongle in order to allow for booting.  Once the machine is booted and the wireless drivers load then the ARM devices could be unplugged.  Check out the hurdles Microsoft had to pass in order to make this work at The Register.

"Such devices, Niehaus said, will have to be certified to run Windows to Go for two reasons, one of which is that in Microsoft's tests external storage ran dangerously hot.

The second reason is that external drives can't be partitioned in the ways Windows 8 requires, thanks to its use of BIOS-replacement Unified Extensible Firmware Interface(UEFI) that is an important contributor to the new OS' faster boot times. Niehaus explained that UEFI means Windows 8 needs four partitions in a disk. One is for recovery purposes, a second for the system, while UEFI uses a third invisible partition of 128MB to help it go about its work. The fourth partition holds the OS and user data."

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