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Oho! Explaining Clean Install for Win10 on Upgraded PCs

I’d been wondering for some time how one could perform a clean install on an upgraded Windows 10 machine without providing a key. Thanks to the nice folks at the Windows Ten Forums, I found a pointer to this Windows 10 Web page entitled “Installing Windows 10 using the media creation tool,” which helps to shed some light on this apparent mystery. Given a copy of the Windows 10 ISO — readily available using the Media Creation tool available from the Download Windows 10 page — simply mount that ISO or use it to create an installable USB flash drive or DVD, then run setup.exe from the root directory of that installation environment (for an upgrade install). To perform a clean Windows 10 installation to replace an upgrade install, you must boot from the upgrade media itself (which works without requiring the key to be manually entered, for reasons I am still chasing down — common sense argues that either Win10 install reads the key from the install being upgraded, or “phones home” to an MS server to obtain that information automatically, and I’ve seen both of those possibilities hazarded by other veteran Windows-watchers like myself).

MS handles a clean (re)install of Windows 10 as a kind of upgrade and retains the upgrade key.

The secret is briefly mentioned at the very tail end of the afore-linked “Installing windows…” page, where the text explaining installation media includes this comment:

Both of these options allow you to upgrade the PC if it’s already running Windows 7, Windows 8, Windows 8.1, or Windows 10.

The two options mentioned are the ISO mount or install from UFD or DVD already mentioned, where a clean install from such an environment is apparently able to extract the key information from the running Windows 10 installation it is about to replace before overwriting (or adding to) the boot/sys volume on the PC that’s acting as the installation target. I’ve tried this now with an install onto an existing Windows partition, as well as an install that begins by repartitioning the boot/sys drive (and volumes) before actually copying files, and can confirm that both methods work.

Among other things, this means you can select a target disk for your new clean install to replace an existing Windows 10 install on a drive different from the one that housed the initial upgrade installation. This can be very handy when replacing the boot/sys drive, or when switching from an older, slower boot/sys drive to a newer, faster one. I’ve also been able to confirm that capability on my Lenovo laptops, both of which made the switch from an older, slower OCZ Agility3 boot/sys drive to a newer, faster Plextor mSATA boot\sys drive as part of the Windows 10 adoption process.

And for those who may be concerned about the old, tried-and-true method of capturing the Windows 10 key, and then reusing it during installation: that still works, too, as I confirmed early in my massive upgrade-to-10 sequence in late August and early September here at Chez Tittel. But it’s nice to know you aren’t forced to capture the key if you don’t want to (I always grab mine as a matter of course, however, being paranoid enough to realize that just because Windows 10 isn’t out to get me doesn’t mean I might not have to recover from a catastrophic crash some time in the future).

Cheers, and Happy Labor Day!