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These two Windows 10 ISO files look identical, but there’s a crucial difference.
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If you’re planning to do a completely clean install of Windows 10, with no trace of the previous operating system or data left behind, you need to start your PC using bootable media and run the Windows Setup program from there.
For most modern PCs, that means getting your hands on a USB flash drive with at least 8 GB total storage capacity and then downloading an ISO file containing the latest installation image. Just make sure you choose the right ISO file.
Microsoft offers downloads of the Windows 10 installer files from multiple sources. But not all ISO files are created equal, as you’ll discover if you compare the setup files carefully.
The screenshot on this page shows the contents of the Windows 10 version 1709 ISO as downloaded from the Visual Studio subscription portal (formerly MSDN) and from Microsoft’s public servers, using the Media Creation Tool. The subscription download is nearly identical, except that it contains the key setup files in WIM (Windows Image) format. The public download supplies the same files in ESD (Electronic Software Distribution) format.
In the most recent version, the ESD file is 3.40 GB in size, while the WIM file is nearly a full gigabyte larger at 4.34 GB.
Why does it matter?
When you use the built-in Windows 10 utility to create a recovery drive, your USB flash drive is formatted using the FAT32 disk format. That configuration is required to install Windows on a modern PC with a GPT partition.
But the maximum file size for a FAT32 drive is 4 GB. So when you download an ISO file from the Visual Studio site, double-click to mount it as a virtual drive, and then drag its contents into the window containing the recovery drive, Windows throws up an error message and refuses to copy that WIM file. It’s too large to fit on your FAT32-formatted drive.
There are several solutions to this problem, including the use of third-party tools. But the easiest solution is to use the Media Creation Tool, which downloads files specifically tailored for use with a bootable USB flash drive in FAT32 format. The ESD file is more highly compressed and stays comfortably under the 4 GB limit.
Using the Media Creation Tool, you can create bootable installation media directly, but I prefer to download the ISO file instead, which gives me the option to mount that file directly or to attach it to a virtual machine.
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Need more details about the right way to do a clean Windows 10 install? See these previous articles: