If you're worried about updating your Mac to macOS High Sierra (perhaps due to incompatible software) or if you're testing out the beta version of macOS Mojave and don't want to risk messing up your system, you don't have to install the update on your Mac at all - you can install it on an external hard drive instead.
Installing the Mac operating system on an external drive is the safest way to get a good look at a new OS without changing anything on your Mac. It'll leave your current configuration intact and allow you to play around with High Sierra or the Mojave beta. (Elsewhere we've explained how to install a macOS beta.)
In this tutorial we explain how to run macOS from a external drive.
What you need
You'll want to use a fast drive if you can - Thunderbolt drives are comparable to the internal drives on a Mac so it's worth investing in a Thunderbolt drive if this is practical. If not, at least make sure that you opt for flash storage.
That drive will need at least 12GB of space, but more is recommended.
For buying advice, take a look at our guide to the best external drives for Mac.
Prepare for the installation
Follow these instructions to prepare your external drive:
- Connect the external hard drive.
- Launch Disk Utility (press Cmd + spacebar and start to type Disk Utility).
- Select the drive in the sidebar (the root drive, not the volume it contains).
- Click Partition.
- Ensure that Partition Layout says 1 Partition.
- Give it a name such as "macOS Mojave".
- Click Options and ensure that Guide Partition Table is selected. Click OK.
- Click Apply and Partition.
Install macOS on the hard drive
Now download the macOS Mojave beta, or a High Sierra installation from the Mac App Store (or whichever version of macOS you require - you can find older versions under the Purchased tab) and follow these instructions to install the beta on the external drive:
- Download the relevant macOS installer from the App Store (or obtain the beta version following our guide mentioned above), and open it once downloaded.
- Click Continue.
- Click Agree and Agree.
- Click Show All Disks.
- Highlight the external hard drive.
- Click Install.
- Enter your Admin password.
- To run the version of macOS on the external drive, open System Preferences > Startup Disk. Select the drive you wish to start up the Mac and click Restart.
- You can also hold down Option/Alt during startup to pick the drive you wish to use to start up the Mac.