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How can I install Windows 7 from an external USB Hard disk?

I format many computers a day and need both 32-bit and 64-bit versions.

I was wondering if I could launch an hypotetic 'live' installer and then load the wanted .iso from my external memory ?

NB: this is not a duplicate of How to setup Windows from an external hard drive?

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Is there any particular reason it needs to be an external USB HDD, as opposed to a flash drive? You can have a 32bit and 64bit Win7 Ultimate install on a USB flash drive. – Oliver G Jul 26 '12 at 10:58
I don't have any pen drives. also: on an external HDD I can copy multiple images (iso) and whatever Service Pack – iAsk Jul 26 '12 at 11:21
In that case, just build the bootable ISO you need including whatever software and update packages you want, and extract it directly onto the root of your external HDD. After that you can just configure whichever computer you're installing on to boot from the External HDD. – Oliver G Jul 26 '12 at 11:28

There are many third party tools available on the internet solving your problem. Some of the tools are:

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You can use YUMI – Multiboot USB Creator for this purpose. Using this tool you can create a bootable drive using for Windows and Linux distros as well.

Details about YUMI: YUMI – Multiboot USB Creator

Since you want to make your external USB drive bootable, you would need to check 'Show ALL Drives?' enter image description here

Other similar tools to YUMI are:

  1. SARDU – Multiboot USB Creator (Windows)
  2. XBOOT – Multiboot ISO USB Creator (Windows)
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I guess it is similar to Unetbootin - – iAsk Jul 27 '12 at 13:33
I think Unetbootin doesn't allow creating multi OS bootable USB drive whereas these tools would allow you to create a bootable flash drive with multiple OSes. You can choose whichever OS you want to install. – Shivaranjan Jul 27 '12 at 14:36

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