You plug the external drive into the target computer, restart, and enter the boot menu (usually F12, ESC, or F2). You select “USB Hard Drive” or the external drive’s name. The computer boots straight into the blue Windows Setup screen — just as if you were using a USB flash drive.

You mount the Windows ISO (double-click it in Windows 10/11) or extract it with 7-Zip. Then you simply copy all files and folders from the ISO to the external drive. That’s it — no special tools needed.

It takes about 10–15 minutes for the copy to finish. When it’s done, your external hard drive is now a Windows installation drive.

You open Command Prompt as Administrator .

Here’s a short, narrative-style guide based on the phrase — told as if you’re the one doing it. You’ve got a laptop with a broken internal drive, or maybe you just want a fresh Windows install without burning a DVD or making a USB stick from scratch. All you have is an old external hard drive and a Windows ISO file.

You install Windows normally, choosing the internal drive (or even another external drive if you want a portable Windows). The external installation drive stays untouched unless you deliberately format it.

Some older computers won’t boot from an external hard drive via USB. And if your external drive is slow (5400 RPM), the initial boot to setup might take a minute or two. But once the Windows installer loads into RAM, speed is fine.