Installing OS X Mountain Lion brings up a message "an error occurred while preparing the installation. Try running again" every time I try to install the upgrade.

Maybe it's time to try the download again. It should be available (at no additional charge) on your App Store --> Purchases page.

Painful, but hopefully the pain is worthwhile to get a flawless system.

TaylorMich:

Or try reboot in Safe mode (holding down Shift-key) and install from there.

Try out the boy´s solution:

http://www.youtube.com/watch?v=zUovYAXCutc

Ensure your clock is set correctly. I'm recovery, open utilities -> terminal and run "date". The date will probably be wrong, you need to set it to the correct date.

I had the same issue trying to reinstall from the OS Utilities boot screen (cmnd +R on reboot). Check your Time Zone in the System Preferences is set correctly, even if the Date Time is correct in the Finder. This fixed my problem.

Maybe this will help or at least give you some ideas:

http://www.reedcorner.net/understanding-upgrade-nightmares/

So mswadling's tip actually solved the problem for me. Somewhere along the line the system clock got reset to 1/1/2001 and the OS X wouldn't reinstall without that clock being corrected. Once I corrected the clock in Terminal, it reinstalled without error

The date being incorrect fixed it for me too.

To tell what your time is set to, enter the following command in the Terminal:

date

Which will give you an output like this:

Tue Jun 25 18:43:41 EDT 2013

To then set the date use this format:

date 0625184313

Where the format is MMddhhmmyy where

MM is two digit month

dd is two digit day

hh is 24 hour hour

mm is two digit minute

yy is two digit year

Though posted somewhere else already, this might be well placed here too.

I had the same problems and want to provide my solution.

Key facts:

- iMac w/ OS X 10.7.5 (Lion)

- Intel Core i5 64bit

- 4GB RAM

Knowledge that i collected during extensive search through the internet:

- ML requires at least 2GB RAM, better have 4GB available

- Requires 64bit System ( Any AppleComputer after 2007 should fit the requirements )

- The target partition on your system should be formatted as "Extended ( Jounaled )" and have at least 15 GB of free memory!

Problem description:

- Starting ML Installation appears to be OK until it begins to prepare.

- After i confirm the "restart" request, some further preparations are done with the message "computer is restarting…" visible.

- After a few seconds the message "An error occured during preparation of installtion. Try again...."

- Inspecting the log shows me: "Couldn't bless HDD <...>"

My solution:

1. Shut down Computer and on restart, shortly after you here that noise before the startup sound appears press and hold simultaneously "cmd+R" and wait until the gray-scale apple logo disappears.

2. Given the selection of time-machine, disk utilities etc. select "Disk Utilities"

3. Select your HDD on the left column of the subsequently appearing window and click on "verify disk permissions" and wait a while

4. If anything except "Permissions appear to be OK" is shown, select "repair disk permissions"

5. When finished select "verify disk" and wait

6. If anything except "HDD appears to be OK" is shown, select "repair disk" and wait

7. When finished, restart computer and again before the startup sound and the gray screen appears press "CMD + Option + P + R" to reset NVRAM/PRAM partition. The computer will restart automatically after est. 5 seconds.

8. Start up normally and attempt installation over your current installation.

My problem arose from corrupted disk permissions, that prevented the ML installer from properly verifying the target disk!

I hope this is of value for someone. 😀

Cheers!

I had the same problem but it was fixed after i went to the disk utility >choose the osx installer on the left hand panel> mount> unmount . DONE . It works perfectly for me

THANK YOU.

After spending ages trying to work out why an old laptop in my company wouldn't restore this finally resolved it for an internet recovery. Genius. 🙂

go to show contents> and find the dm file and copy to desktop and use that file

It’s amazing the boy’s fix still works 6 years later lol