Mountain lion

None, if you do it correctly:

How to Install Lion/Mountain Lion Successfully - You must have Snow Leopard 10.6.6 - 10.6.8 Installed

A. Repair the Hard Drive and Permissions:

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. Then select Disk Utility from the Utilities. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list. In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive. If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer. Now restart normally.

If DU reports errors it cannot fix, then you will need Disk Warrior (4.3.) if DW cannot fix the drive, then you will need to reformat the drive and reinstall Snow Leopard.

B. Make a Bootable Backup Using Restore Option of Disk Utility:

1. Open Disk Utility from the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Check the box labeled Erase destination.
5. Select the destination volume from the left side list and drag it to the Destination entry field.
6. Select the source volume from the left side list and drag it to the Source entry field.
7. Double-check you got it right, then click on the Restore button.

Destination means the backup volume. Source means the internal startup volume.

C. Important: Please read before installing:

1. If you have a FireWire hard drive connected, disconnect it before installing the update unless you will boot from this drive and install the update on it. Reconnect it and turn it back on after installation is complete and you've restarted.
2. You may experience unexpected results if you have installed third-party system software modifications, or if you have modified the operating system through other means. (This does not apply to normal application software installation.)
3. The installation process should not be interrupted. If a power outage or other interruption occurs during installation, use the standalone installer (see below) from Apple Downloads to update. While the installation is in progress do not use the computer.

D. To upgrade to Lion/Mountain Lion:

1. Purchase the Lion/Mountain Lion Installer from the Mac App Store. The download should start quickly. Lion is nearly 4 GBs so a fast internet connection is essential. Download time could run upwards of 4 hours depending upon network conditions and server demands at the time.
2. The installer will run automatically after the download is completed. Click on the Install button to begin the upgrade.
3. Follow instructions for installation.

I don't understand 'B'. It lappears that you're saying we should copy our boot partition to a destination, but first we have to delete everything that's on the destination partition. That can't be correct. And is it necessary to copy the entire boot partition?

When I got my 8-core Mac Pro, the first thing I did was erase the hard drive, divide the disc into two partitions, and re-install the OS onto one of them. I use my non-boot partition for storage, so I'd like to install Mountain Lion onto that. First I'll get rid of any unnecessary files, and defragment. You mention a stand-alone Mountain Lion installer, and say 'see below'--but I don't see. I'll have to look around to find it I guess. Early in the OS X cycle ( I think it was Jaguar), I tried upgrading an existing OS, and the new OS trashed the existing install. Good thing I had that extra partition so I could reinstall the old one and start over.

The trouble with an online OS install is that it may automate more than I want, or expect. I have learned to avoid automation of this kind.

Greg

---

8-core Mac Pro; OS 10.6.8

That's correct. You cannot create a clone on a drive that is not freshly erased. Disk Utility will automatically erase it if you don't. Yes, it's necessary to do as it says.

As much as you may like, you cannot install Mountain Lion onto the other partition without first erasing it unless it already has OS X installed on it, in which case you can upgrade that system to Mountain Lion.

Nevertheless, you posed the question. I've outlined the answer. If you wish to do something differently, then that's certainly your option.

Thanks, Kappy. I've looked around and see what you're talking about. I am amazed how Apple has taken a simple OS install and turned it into the most convoluted thing possible.