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tjmarine6492 remedy worked for me. Downloading now.

"Ok, I was having the same issue, tried everything and nothing worked. I finally stumbled upon a post from ifixit or mac fixit and they said:

"Another approach that is working for people is to clear out caches and other temporary files used by the App Store. To do this, open the Terminal utility (in the /Applications/Utilities/ folder) and run the following command:

open $TMPDIR/../C

A Finder window will appear that shows a number of folders. Locate and remove the folder called "com.apple.appstore," and then try launching the store and purchasing Mountain Lion again."

I tried this and bam! It started downloading with no issues. All I did was remove the folder to the trash and then emptied the trash to make sure the file was NOT present anymore. I hope this helps."

I had this happen in Maui with an unstable internet connection while trying to download the latest Xcode - your idea worked perfectly with no negative sideeffect, other than losing the 1% of xcode update (1.56GB!!!) that had downloaded. THANKS!

Ken

Boom! Works like a charm!! Thanks a lot 🙂 I created this ID to thank you 😉

Here's the clear answer.

Resolution

If the customer has Intego VirusBarrier X6 installed

1. Open /Applications/VirusBarrier X6.
2. Click on the "Surf" button.
3. Click on the "Ad Banner Filter" tab.
4. Click "OFF" next to "Banner advertisement filter".
5. Start the computer in Safe Boot, then restart the computer into normal mode.

If the customer does not have Intego Virus Barrier installed

1. Verify that the date and time is correct on the computer.
2. Safe Boot once, then restart normally, then try downloading again.
3. Try with a different user account if the issue persists

If the issue persists

1. Install Lion onto an empty partition (internal or external).
   Note: You can Use Disk Utility to live partition the internal drive and create a second partition large enough to install Lion.
2. Start from the clean installation of Lion.
3. Download Mountain Lion from the Mac App Store. Note that the original startup disk can be set as the destination for Mountain Lion during the installation process.

If Safe Boot cannot be used

1. Open Terminal.
2. Type:
open $TMPDIR/../C

3. In the Finder window that opens, drag the "com.app.appstore" directory to the Trash.
4. Open the App Store and download Mountain Lion.

Press & hold down " Shift " key and start the device