Are USB2 external drives fast enough to run a Windows operating system from?
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?
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.
-=EDIT=-
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.
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).