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I'm interested in creating a portable Windows installation and my PCs only have USB2. I never did this kind of thing before so I am concerned about a USB flash memory stick or an external hard drive not being fast enough to be useful. Can you run Windows from a USB2 drive?

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up vote 4 down vote accepted

USB2 theoretical maximum speed is 480Mbps (mega-bits per second) which equates to ~60 megabytes per second. With other devices and protocol overheads the effective speed of USB2 is somewhere around 40 megabytes per second.

This is a workable speed, not perfect, but definitely usable.

Flash drives (especially cheap ones) will usually be half that speed.

One problem with booting from USB is that most operating systems will not work happily with it without some fancy footwork going on. Actually using an external drive to boot Windows is not supported and certainly isn't an easy thing to do.

From Raymond Chen:

Another reason not mentioned in this paper is that during any hot-plug operation, the USB bus is completely reinitialized. Windows really doesn't like it when it loses access to its boot device. Imagine, you plug in a USB camera, the USB bus reinitializes, Windows loses access to the boot drive, and oops the kernel needs to page in some data and it can't.


Windows 8 Enterprise supports Windows To Go which, according to Microsoft is

your own fully manageable, corporate image installed on a bootable certified USB drive. It is a new feature of Windows 8 Enterprise to help businesses address a wide range of mobility and travel light requirements

Microsoft has a guide (Word Document) on how to create a Windows To Go drive.

This is only supported by Windows 8 Enterprise, not any other edition.

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It is supported in W8 though? – Mr. Boy Mar 23 '13 at 12:49
@John updated with Win8 option – Mokubai Mar 23 '13 at 13:52

As far as speed is concerned, yes, it ought to be enough, even if the boot itself will be really, really slow until the caching catches up.

But you will have problems with everything else: software, hardware, BIOS, and license.

  • BIOS: it's not a given that the USB2 device will be properly "seen" even if it supports a suitable identification (I'm thinking Cruzer U3). I have seen one BIOS - I believe it was on a Gigabyte motherboard - which allowed bus separation on one USB device, allowing it to be used as a disk for all intents and purposes. The fact that it required a BIOS hack to make it work properly makes me think that you're liable to encounter problems without.
  • hardware: Windows configures itself upon installation, adapting to the underlying hardware. If you plan on plugging the boot disk on different devices, you might hit some unforeseen unsolvable incompatibility, maybe even preventing boot itself.
  • software: whenever the USB layer is reinitialised (and sometimes it can apparently do this all by itself), unless the BIOS is capable of preventing this (see above), the disk will temporarily "disappear", which could wreak havoc at kernel level; I don't think its programmers made allowances for such a situation.
  • license: check whether it's possible to effectively move a Windows installation from one PC to another. Under some multiple-license schemes, I would be allowed to do this as long as all the machines are within my organisation and covered by the same MOLP terms (for example a sort of testing, recovery, verification or installation "disk" with a whole bunch of appropriate software preinstalled). I do not know whether such terms are provided by any Microsoft license.

If you plan on replacing a boot device with an USB device (because it's easier to upgrade, sturdier, no moving parts, less power hungry, etc.), you might be better served by ATA Compact Flash (CE-ATA) or SSD. I have seen kiosk installations (albeit running Linux) where the boot disk was actually an external CF (later a SSD) mounted on an ejectable bracket, so that "system upgrade" could be performed by DHL'ing a replacement boot disk to the customer, who could do the upgrade itself (we also had some theft problems due to the ease of removal of the disk, which you might need to consider).

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Sounds like a VM on the external drive would be much better then? – Mr. Boy Mar 23 '13 at 14:38
I use currently several VMs - one, I confess, with "licensing problems" (it's a clone of my old PC, which had a license before being scrapped) on an external USB2 drive. Won't call any of them "blazing fast" (my next PC will have an USB3 for that purpose alone), but they work perfectly. VMware VMDKs, created with P2V or from scratch from . – lserni Mar 23 '13 at 22:54

It would be fast enough (it only matters for startup because after that the OS is mostly running in memory), but moving the USB stick between PCs would violate the licence terms with Microsoft. An Operating System is licenced for the computer it runs on. If you are sticking the USB stick into other computers it is not licenced for those computers. It also may not run properly.

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By run properly, if the systems are not identical, when you boot, it will be trying to load the appropriate drivers for the hardware of the computer it is currently running on. It will also be disabling the hardware from the previous computer. LOTS of read writes during boot so definite slow boot there. Also, yes the EULA prohibits this as well. I'm not sure how the read/writes will affect the longevity of the device if you use a flash drive. – Blackbeagle Mar 23 '13 at 12:40
I have one license key which is allowed to be installed on up to 10 PCs so presumably I could share one installed HD between 10 PCs? – Mr. Boy Mar 23 '13 at 12:50
That sounds like a volume licence. I don't know licencing very well but I think so. You might still have issues with different hardware/drivers when moving between PCs though, unless the PC hardware is identical. I know with servers you can remove the HDDs and stick them into identical model servers without issue. – MrVimes Mar 23 '13 at 12:54

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