Can't open Excel files after installing OS X 10.8 Mountain Lion on my Mac. HELP PLEASE!

Just go to apple store and let them reinstall everything for you. its free and hassle free.=) Warranty or without warranty.

You cannot do what you want without the original installer disc that came with the computer or a later version of OS X installer, unless the computer came originally with Lion or Mountain Lion.

Follow these instructions step by step to prepare a Mac for sale:
Step One - Back up your data:
A. If you have any Virtual PCs shut them down. They cannot be in their "fast saved" state. They must be shut down from inside Windows.
B. Clone to an external drive using using Carbon Copy Cloner.
1. Open Carbon Copy Cloner.
2. Select the Source volume from the Select a source drop down menu on the left side.
3. Select the Destination volume from the Select a destination drop down menu on the right
side.
4. Click on the Clone button. If you are prompted about creating a clone of the Recovery HD be
sure to opt for that.
Destination means a freshly erased external backup drive. Source means the internal
startup drive.
Step Two - Prepare the machine for the new buyer:
1. De-authorize the computer in iTunes! De-authorize both iTunes and Audible accounts.
2, Remove any Open Firmware passwords or Firmware passwords.
3. Turn the brightness full up and volume nearly so.
4. Turn off File Vault, if enabled.
5. Disable iCloud, if enabled: See What to do with iCloud before selling your computer.
Step Three - Install a fresh OS:
A. Snow Leopard and earlier versions of OS X
1. Insert the original OS X install CD/DVD that came with your computer.
2. Restart the computer while holding down the C key to boot from the CD/DVD.
3. Select Disk Utility from the Utilities menu; repartition and reformat the internal hard drive.
Optionally, click on the Security button and set the Zero Data option to one-pass.
4. Install OS X.
5. Upon completion DO NOT restart the computer.
6. Shutdown the computer.
B. Lion and Mountain Lion (if pre-installed on the computer at purchase*)
Note: You will need an active Internet connection. I suggest using Ethernet if possible because
it is three times faster than wireless.
1. Restart the computer while holding down the COMMAND and R keys until the Mac OS X
Utilities window appears.
2. Select Disk Utility from the Mac OS X Utilities window and click on the Continue button.
3. After DU loads select your startup volume (usually Macintosh HD) from the left side list. Click
on the Erase tab in the DU main window.
4. Set the format type to Mac OS Extended (Journaled.) Optionally, click on the Security button
and set the Zero Data option to one-pass.
5. Click on the Erase button and wait until the process has completed.
6. Quit DU and return to the Mac OS X Utilities window.
7. Select Reinstall Lion/Mountain Lion and click on the Install button.
8. Upon completion shutdown the computer.

*If your computer came with Lion or Mountain Lion pre-installed then you are entitled to transfer your license once. If you purchased Lion or Mountain Lion from the App Store then you cannot transfer your license to another party. In the case of the latter you should install the original version of OS X that came with your computer. You need to repartition the hard drive as well as reformat it; this will assure that the Recovery HD partition is removed. See Step Three above. You may verify these requirements by reviewing your OS X Software License

Libre Office works fine and is compatible with Microsoft office documents.

Here's a good start.

Plug your Lacie drive into someone else Mac or PC using MacDrive Pro and see if it shows up.

These ares more of a work around, but you can get a top notch open source free word processor at OpenOffice.org. Fully compatible with Microsoft.

Or, if you have a MS SkyDrive account. It will open MS Office docs that are uploads and stored there. This the new MS office will be intergrated. Clo

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You should be able to install Mountain Lion using the Mac App store. Check the applications folder to find the App Store.

Make sure to back up your files before upgrading. After backing up, you will purchase the upgrade from the App Store. Once it has downloaded you'll be walked through the steps to upgrade.

Hope that helps!

Hello Patrick,

The most common problems I've experienced with Mountain Lion are the following:

• Reduced battery life. I've noticed about an hour to an hour and a half cut off from my battery life of my Macbook Pro. I'm not sure if this applies to you since you have a Macbook Air, but my recommendation is to try a reinstall if you experience battery problems and turn off some of the extra features like Twitter integration and iCloud when you're not using them.
  
• Safari is crashing. Safari is a browser that used to have little respect but now has actually gotten pretty decent. However, with Mountain Lion it actually crashes a lot more than usual. I don't have a fix for it offhand, but I would recommend just using Chrome (which I use anyways) or Firefox. And stay away from Explorer!
  
• Free upgrades to Mountain Lion. When I got my free upgrade to Mountain Lion (if you purchased a Mac after June 11, 2012 you can get one too) it took Apple about 5 days to upgrade me to Mountain Lion. This is a super low priority problem, but it's still something I hope they're working on.
  
• Notes synchronization. I have answered this on FixYa already, you can find it on the site.
  

Best of luck.

You must have the following computer to download Mountain Lion:

iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
Xserve (Early 2009)

Once that is done, you must be updated to the most recent version of either Snow Leopard or Lion. If you don't have either of these operating systems (which is highly unlikely considering the other computers should have them, but I digress) then you will need to get them before Mountain Lion.

After that, go to the Apple Store and download Mountain Lion!