How to Safely Upgrade to OS X Mountain Lion 10.8 (Developer Preview)
Important: This guide covers the Developer Preview (DP1) of Mountain Lion. The final release may differ. The operating system is unstable and intended only for testing. Never use it as your main system. Proceed at your own risk.
Before You Begin – Is Your Mac Compatible?
OS X Mountain Lion requires a Mac with a 64-bit kernel. The following models are supported:
iMac (mid 2007 or later)
MacBook (13-inch Aluminum, 2008), (13-inch, Early 2009 or later)
MacBook Pro (13-inch, Mid-2009 or later), (15-inch, 2.4/2.2 GHz), (17-inch, Late 2007 or later)
MacBook Air (Late 2008 or later)
Mac Mini (Early 2009 or later)
Mac Pro (Early 2008 or later)
Xserve (Early 2009)
If your Mac is not on this list, Mountain Lion may not run at all.
What You Need
A compatible Mac with enough free disk space (at least 20 GB recommended)
The Mountain Lion Developer Preview installer (see Step 1)
A separate partition (created during the guide) where the preview will be installed
Step 1: Download the Mountain Lion Developer Preview
The original guide linked to an external source. Because the Developer Preview is no longer officially distributed, you will need to locate the “Install OS X Mountain Lion Developer Preview” application (often in .dmg format). Search for “Mountain Lion DP1” or the build 12A128p from trusted archives.
Once downloaded, make sure the installer is in your /Applications folder (if it’s a .dmg, mount it first).
Step 2: Create a Separate Partition (Target Partition)
Installing the preview on its own partition keeps your current system untouched. To do this:
Open Disk Utility (from
/Applications/Utilities/).Select your internal hard drive (the whole disk, not a volume).
Click the Partition tab.
Click the + button to add a new partition.
Set the following:
Name:
Mountain Lion DP(or any name you like)Format:
Mac OS Extended (Journaled)Size: At least 20 GB
Click Apply, then Partition. Wait for the process to finish.
You now have a safe, isolated space to install the preview.
Step 3: Install Mountain Lion
Mount the Developer Preview installer (if not already mounted).
Double‑click the “Install OS X Mountain Lion” application to start the installation.
When the installer asks you to select a disk, click “Show All Disks…” (if available).
Choose the partition you created in Step 2 (e.g.,
Mountain Lion DP).Click Install and wait. The initial phase takes about 6 minutes; then the Mac will automatically restart.
Step 4: Wait for the Post‑Restart Installation
After the first reboot, the installer will continue. A progress bar will appear, and the system may sit on a grey screen for a while. This phase can take up to 30 minutes – be patient and do not force a restart.
Once completed, your Mac will boot into the Mountain Lion Developer Preview setup assistant.
Step 5: Test Safely
You now have Mountain Lion installed on its own partition. To switch between your normal system and the preview:
Restart your Mac and hold the Option (⌥) key.
Select the startup disk you want.
Your original files, applications, and settings remain untouched on the main partition. Feel free to test Mountain Lion without fear of breaking your daily environment.
Video Guide
A video walkthrough of the process is available here:
Watch on YouTube (Embed no longer supported on all platforms – click to view)
What Works in This Developer Preview
Mail
Notes
iCloud
FaceTime
iMessages
Safari
Skype
Notifications
Bluetooth
Wi‑Fi
Most other built‑in apps
(Compatibility may vary depending on your specific Mac model.)
A Small but Welcome Change
Apple listened to user feedback: the “Reopen windows when logging back in” checkbox now remembers the last state you chose, instead of always defaulting to checked. A tiny tweak that many power users will appreciate!
Final note: This guide is based on a community tutorial originally published in 2012. The Developer Preview is no longer receiving updates, and some download links may be broken. Use it for historical interest or as a template for dual‑booting beta operating systems.
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