When the GWX app starts downloading the Windows 10 installation setup files, a hidden folder named $Windows.~BT is created on the root of system drive (%SystemDrive%), i.e. C:\$Windows.~BT folder. The folder contains various files necessary for installing and upgrading existing Windows system to Windows 10, including the Install.ESD. The files are initially downloaded into %WinDir%\SoftwareDistribution\Download\ folder, with ESD in complete version name before it’s encrypted and sent to $Windows.~BT hidden folder. The file name of the downloaded ESD file has the format similar to 10240.16384.150709-1700.th1_CLIENTPRO_RET_x64fre_en-us.esd (different language has different language code), confirming that the Windows 10 Build 10240 is indeed the final gold RTM release version. In fact, if you grab the ESD downloaded and convert the ESD to ISO image, you will get the identical Windows 10 RTM ISO image which Microsoft released to OEM manufacturers.
Though Windows 10 RTM is build 10240.16384, but apparently GA build, the version that you’re getting after installing and upgrading to Windows 10, is not. Since Microsoft compiled the Windows 10 Build 10240.16384 on July 9, Microsoft has been working hard to fix any bugs and security vulnerabilities that are discovered since then. As the result, GWX (Get Windows 10) app not only downloads the Windows 10 RTM setup files, but also various updates which are released as ZDP (Zero Day Patch, also known as Day One Patch), device drivers for the system, and OEM-specific apps if you’re using branded PC, which altogether could takes up several hundreds megabytes or even several gigabytes of disk space.
In fact, the setup.exe in the $Windows.~BT folder which likely triggers the setup process has file version of 10.0.10240.16394 rather than 10.0.10240.16384. After upgrading to Windows 10 with all patches and updates (such as KB3074683) installed automatically, while the major version of Windows 10 stays at 10.0.10240, the delta number has increased from 10240.16384 to 10240.16393 (10240.16393.th1_st1.150717-1719) with UBR at 16405. Some executable such as Explorer.exe has the file version of 10.0.10240.16405. In fact, some has identified the build released on July 29th as GA build, which reads as 10240.16405.150725-1815.th1.
What if your $Windows.~BT folder total file size is just 129MB or 140MB and stuck there? Fear not. Take a look at %WinDir%\SoftwareDistribution\Download\8f36b36c5f97d84a69eedf4ec27435ec folder, and check if there is a file with the name formatted as BITXXXX.tmp with XXXX as random characters or numbers. The file may be several GBs in size, which provides evidence that download of Windows 10 is still in progress silently in the background.
While download of Windows 10 installation files may be completed ahead of official release date, but it won’t start the installation by running the accompanied setup.exe file. Instead, the following error is returned:
--------------------------- Install Windows --------------------------- Windows was unable to locate required installation file [boot.wim]. Verify that the installation source is valid, and restart the installation. Error code: 0x80070002
Microsoft stated that GWX app will notify users when Windows 10 is ready to be installed, which should be happened in a few hours for systems that have completed the downloads as Windows 10 GA date approaching.
Update 3: Microsoft released official Windows 10 Media Creation Tool to upgrade to Windows 10 or download ISO.
Update 1: Windows 7 and Windows 8.1 starts to prompt users to upgrade to Windows 10.