Hi,
I've experienced this problem on my personal laptop at home and i've been able to resolve the issue - but I still have no idea what causes it. Hopefully it will help some of you.
This solution applies to those of you that have Steam stuck saying the download is at some percent complete downloading but refuses to go any further no matter what you do. If this does not describe your problem then I doubt these steps will help you.
Please note that these are not official steps for Steam and it requires you to know how to use Windows Explorer (the file browser, not the web browser) and a text editor such as Notepad, if you are not confident with such things then please do not attempt these steps.
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It appears that some how Steam is getting to where it fails to validate one or two depots and this aborts the whole download process.
1. Shut down the steam client.
2. Look in your steam log files, find content_log.txt and review it. See if you find one or two specific depot id's that report something along the lines of not being able to get a validation on.
For example, a line like this:
[2014-09-17 01:48:31] HTTP (CS,100) - valve18.cs.steampowered.com (valve18.cs.steampowered.com): AuthenticateDepotID (24012) - ERROR: Failed to get AppOwnershipTicket from Steam
Means that it's failing to validate 24012.
Once you've worked out what the failing depot id's are (email them to
support@dovetailgames.com please!) you can tell Steam not to bother with them.
3. In your SteamApps folder, you'll find "appmanifest_24010.acf" - open this in notepad and be careful what you do in this file just to make sure it stays "good".
You're looking for a line similar to this:
"24021" "6558406753873641691"
(where in my example "24021" is replaced with the depot id you're searching for - ignore the big number on the right.
4. When you find that line, REMOVE IT entirely (including the big number) so that the list has no gaps or formatting differences etc.
5. Save the file, restart Steam.
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This cured it on my laptop and once steam was restarted it resumed download and finished quickly.
I hope to have a better (and more easy to understand) answer from Steam soon, but in the mean time hopefully this will get some of you experiencing this problem back up and running.
Apologies for the inconvenience.
Matt.
Dovetail Games.