Tjyylli

I just bought a HP Pavilion Notebook 15 with a hard disk drive and I took the SSD out of my old Lenovo Thinkpad T510 with Windows 10 installed, that I have been using for years, and swapped them.

I wanted to make fresh install of Windows 10 on the SSD, however when I started laptop I realized it is working as I left it in the older laptop. So I can continue without installing anything.

How is this possible? I always understood that you can’t run Windows on a different motherboard than the one it was installed on. Should I still do a fresh install of Windows 10?

If Windows comes up as activated, then you apparently have a
retail version of Windows, also called "full version".

If you have purchased a retail copy of Windows 10, then you're entitled to transfer the license to another device multiple times. The same is true when upgrading a retail copy of Windows 8.x or Windows 7 to Windows 10, and you want to move the product key to another device.

After you move your license several times, Windows may give you an activation error and ask you to call Microsoft to activate your computer. Microsoft’s representatives will allow it, they just want to make sure you aren’t installing the same license on multiple PCs at a time.

On the other hand, when purchasing a new computer with Windows 10 pre-installed, you're getting an Original Equipment Manufacturer (OEM) license. This means that you're not allowed to transfer the product key to another device (under the Microsoft rules). The same will be true when upgrading a device that came with Windows 8.x or Windows 7 pre-installed.

As you will never boot this disk again on the old computer, the Microsoft
servers will not signal a possible activation abuse, so no problems
for you and you are completely legit..

You can run Windows on different motherboards than originally installed on in many cases. I've successfully transplanted motherboards of different types when repairing failures.

Windows normally barfs if the motherboard changes because if driver issues. If the hardware is similar enough, it can continue to work think, of a basic system using all Intel chipsets.

I postulate that the systems had enough common hardware for windows to start and get a network connection after which it can update/tweak itself with drivers it can download.

Windows comes preinstalled with loads of generic drivers, or your existing drivers might be a close enough match for the hardware that they are included in the same driver. When windows starts it redetectes its hardware and loads the most suitable driver it can find. Either way it's best you download and install latest drivers for your new hardware, you could be using ancient generic drivers out of the box!

Given the hardware change you need to reactivate windows or insert a new key (I recently bought a w10 pro key £3.49 off ebay!). If you need to reactivate it might fail from the computer, but ringing the automated activation line generally works.

So long as your windows is activated, windows updates work and you install all drivers it'll probably work fine, albeit with some registry and windows folder bloat.

As a random side note you can inject drivers into windows before moving the disk between computers using PNPUtil. That way windows starts with the latest drivers. It also means you can install storage drivers and such that would otherwise prevent windows booting on different hardware.