It seems that many people are having a big problem with the Blizzard Background Dowloader when trying to update to patch 4.0.0 from patch 3.5.5; I feel your pain, as I was as well. With slow download speeds placing my patch time at around 4 DAYS, I thought I’d hunt around for a solution. And lo, the solution is easy!
1. Start your background downloader by either starting your game or double-clicking ‘Background Downloader’ in (World of Warcraft folder location)> World of Warcraft.
2. Let the Background Downloader run until you’ve achieved at least 1% of the download. If your Background Downloader is hidden, double-click on the Blizzard icon in your system try (bottom right of your screen) to embiggen it. Once you’re at 1% or more, right-click on the icon to exit out of the Background Downloader.
3. Open the folder (World of Warcraft directory)>World of Warcraft>Cache and locate the BackgroundDownload.torr file. If you can’t see the “.torr” part, that’s okay.
4. Open your favorite torrent downloader. I like BitComet, myself.
5. Pull the BackgroundDownload.torr file into BitComet. Click ‘Download Now’.
6. Enjoy the lightening-fast download speeds.
7. When the file is complete, locate the BackgroundDownload file in Cache and move it to the folder labeled ‘Updates’.
8. The End.