can i install operation system on external hard drive?

Yes you can do that, but it takes a fairly large external hard drive. You would also need to always have it plugged in when running it. There's also some issues that your computer may have with running a separating OS over your old one. However, it is possible. Good luck!

yeah its perfectly possible to install an Operating systems on and external drive (USB drive) and boot from the external drive. Even some of the operating systems comes on live cd , where entire operating system is virtually installed on a DVD. Anyway, below are two link which might be helpful for you to know how to do it and how it works: http://www.microsoft.com/whdc/archive/usb-boot.mspx http://www.pcstats.com/articleview.cfm?articleID=1676 http://www.suite101.com/content/can-you-boot-from-external-hard-disk-drive-a84327 Hope these links helps you in case you need to install os on your USB. All the best Cheers

I've never do that. In my opinion, it will be impossible, at least it will be very very hard work. I think you may install both operating system easily by installing the other one in the other drive on fixed drive. You have to have two hard drive or one hard drive which partitioned at least two. For example, you have installed Windows XP in drive C: then you may choose drive D: to install your Vista. It will be work. You just choose one of them to be your operating system when booting. I hope this will help you.

well.. Surely we can install OS on external hard drive.. But i guess its not that easy. I had tried one or two time but didn't got success.. By the way you can format the drive which having xp operating system using vista CD. Its easy too.

Yes, but you can only use it on pc where you made your installation, because some hardware and peripherals that read when you installed your OS.