If you used Software Update to download macOS 27 Golden Gate, this section does not apply to you. If you downloaded a standalone installer, bear in mind that if you do an in-place upgrade (whether to your internal SSD or an external SSD), the installer is deleted from /Applications once the process is finished.
I presume the reason for this is that, once macOS 27 is installed, Apple wants to give you back the 21 GB or so of disk space the installer was taking up, since you likely won’t need the installer in the future (and can download it again if you do). But what if you want to install macOS 27 on multiple Macs—or reinstall it later on the same Mac—without having to download that huge installer again? I’ll tell you what: you’d better put that installer in a safe place before you run it! If it’s anywhere else—your desktop, ~/Downloads, or wherever—it won’t be deleted after it runs.
So, before you do anything else, I suggest that you either ⌘-drag the installer to move it from /Applications to another location, or Option-drag the installer to copy it to another location—preferably another volume—so if it is deleted, you’ll have a spare. Or create a bootable installation volume, as below.
Make a bootable installer volume
The macOS 27 installer doesn’t include or require boot media. But you might want to have a copy of the installer on a bootable disk for one of these reasons:
- As a backup, so you don’t have to re-download it if you need it in the future
- To simplify installing macOS 27 on multiple Macs
- To facilitate downgrading to your previous version of macOS should that become necessary, in which case you’d want a bootable installer volume for the old version, not for macOS 27
If none of that applies to you, skip to the last section.
To move ahead with a bootable installer volume, you’ll want a flash drive with at least 32 GB of free capacity. (A 32 GB USB flash drive can be found for under $10 online or at your favorite local electronics store.) Then you follow a simple procedure to put a bootable installer on that drive and, when the time comes, start from that volume and run the installer.
Next, you’ll use a command-line tool called createinstallmedia that is buried inside the installer (and thus is a method that Apple officially sanctions). Using this tool requires a visit to Terminal, but it’s really a matter of copying and pasting one line from Apple’s website into Terminal and replacing the name MyVolume with the name of the drive you’ll be using.
But there’s a catch, in that a bootable installer requires a full macOS installer to work with, and in some cases that may not be what you get when you download macOS 27.
Update your backup
You should already have a complete backup of your startup disk. If you did that just minutes ago—and your backup includes the macOS 27 installer file located outside the /Applications folder—then you can move along to the next step. Otherwise, I suggest updating your backup before running the installer.
With the installer safe, you can run it.
