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.
diskpart list disk (find your external drive – be careful not to pick the wrong one ) select disk 2 (for example) list partition select partition 1 active exit install windows from external hard drive
First, you open and delete all partitions on the external drive. Then you create a single primary partition, format it as NTFS , and mark it as active (so the computer knows it can boot from it). You install Windows normally, choosing the internal drive
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. Then you create a single primary partition, format
You open Command Prompt as Administrator .
You plug the external hard drive into a working computer. It’s 500 GB, mostly empty. You know Windows setup won’t need all that space, but the drive has to be bootable.
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.