In the lead-up to Windows 10's release this summer, enterprising techies could download free preview builds of the operating system through Microsoft's Insider program. A new Windows OS for free was a pretty tempting carrot. However, Microsoft understandably shut off this spigot once Windows 10 was released, since it didn't want everyone to keep grabbing the test builds. Now that the dust has settled, Microsoft has turned the spigot back on, with one important change: the device you're installing the build on must already have an activated copy of Windows 10 associated with it.
Judging by Microsoft's wording -- "You will need to install the Windows 10 Insider Preview build on a device that has been previously activated with Windows 10 or with a Windows 10 product key" -- the device doesn't have to have Windows 10 on it when you install the Insider build on it. The device just needs to have had an activated copy of Windows 10 on it at some time, which would include previous Insider builds. So if you have Windows 10 or used to have an Insider build of the OS on your machine, you should be able to install these new builds, even on a blank drive.
As we've mentioned before, activation creates a unique fingerprint for your device that's stored in Microsoft's cloud, and it only needs to be done once. And under ordinary circumstances, this happens automatically during installation. Microsoft will then recognize the device by its fingerprint in the future to authorize further activations, if necessary.
If your device activated a copy of Windows 10 in the past, you can grab Insider builds from Microsoft's website. These are ISO files, which you can burn to a DVD or install on a USB stick using Microsoft's official Windows USB/DVD Download Tool. Be aware that using the USB tool will format your USB thumb drive, so make sure that you move all your important stuff to another location first.
Windows upgrades get an upgrade
In related news, Microsoft recently announced an upcoming change to the Get Windows 10 campaign (where qualifiying users of Windows 7 and Windows 8.1 can upgrade to Windows 10 for free until July 29, 2016): You will be able to activate the upgrade with a Windows 7 or Windows 8.1 product key, rather than having to install the upgrade on top of Windows 7 or 8.1 first.
That means you can do a fresh install of Windows 10 on a formatted storage device and use one of your older keys to activate it. Just so we're clear, the Get Windows 10 version of Windows 10 is different from the Insider Build. Get Windows 10 delivers the retail version of Windows 10, while the Insider system is for beta testing versions of Windows before they reach the general public. The new version of Get Windows 10 should be available before the end of November.