How can I install Windows 7 on external HDD.?

I have to agree that it seems strange to want to do so. I would not suggest as your main operating system. If you really want to install it on an external drive. I would use your main computer that is connected to the external hard drive.

Install a product called VMWare or Microsoft Virtual Server, and create a virtual machine on the external hard drive. That way you can load that version of Windows if you want to do that on your main machine, without being concerned for the underlying hardware it runs on.

Why could you desire to have the OS on achronic it particularly is now no longer in direct touch with the hardware? in case you attempt this and then get rid of the HDD your chip-set won't discover the OS and your kit will fail besides! and likewise you would be able to't run 2 OS on an same pc at a similar time so the interior attain OS could have administration of the hardware and diverse OS heavily isn't used. IMHO a complete wast of attempt and time

Why would you want to have the OS on a drive that is NOT in direct contact with the hardware?

If you do this and then remove the HDD your chip-set will not find the OS and your system will fail to boot! and you cannot run TWO OS on the same computer at the same time so the local OS will have control of the hardware and the other OS will not be used.

IMHO a total wast of time and effort

connect the harddisk to a computer

boot the computer from the cd

and when you get the disk option(may be after 2 3 steps in the installation wizard...which has format,rename etc in it)

select your external harddisk to install the file

and click on format

click on install

...wait for a while and its done