Is there any way to install macOS Sierra on a Mac that's officially too old to make the update?
Mac updates its macOS (previously Mac OS X) desktop and laptop operating system once a year, like clockwork.
The yearly macOS updates are free to install from the Mac App Store (read about How to update macOS here), but not everyone can join in: part of the yearly ritual involves the announcement of which Macs will certified to run the new software, and as was the case with macOS Sierra, certain models are deemed too old and underpowered to make the grade.
Apple advises that macOS High Sierra will run happily on a late 2009 or later MacBook or iMac, or a 2010 or later MacBook Air, MacBook Pro, Mac mini or Mac Pro. If you've got an older machine than that, the Mac App Store simply won't let you install the upgrade file.
But all is not. A patch tool written by a developer known as DOSDude1 enables you to install macOS High Sierra on an older Mac: we can go back as far as early 2008 models. It's unlikely to run like a dream, which is why Apple advises against this course of action. But you should be able to achieve tolerable performance. (Dosdude1 wrote a similar patch for MacOS Sierra in 2016).
For information about Apple's terms and conditions for using High Sierra read: Should you agree to Apple's terms and conditions
How to install MacOS Sierra on an old Mac: What you need
Specifically, this hack works on Macs with Penryn architecture: early-2008 or newer Mac Pro, iMac, or MacBook Pro (MacPro 3,1 and 4,1, iMac 8,1 and 9,1, MacBook Pro 4,1, 5,1 5,2, 5,3, 5,4, and 5,5); late-2008 or newer MacBook Air or aluminium unibody MacBook (MacBookAir 2,1, MacBook 5,1); early-2009 or newer Mac Mini or white MacBook (Macmini 3,1, MacBook 5,2); or early-2008 or newer Xserve (Xserve 2,1).
DOSDude1, who wrote the patch for this hack, stresses that the following Macs are not compatible: 2006-2007 Mac Pros, iMacs, MacBook Pros and Mac Minis (MacPro 1,1 and 2,1, iMac 4,1, 5,1, 5,2, 6,1 and 7,1, MacBook Pro 1,1, 2,1, and 3,1, Macmini 1,1 and 2,1), although the 2007 iMac 7,1 is compatible if the CPU is upgraded to a Penryn-based Core 2 Duo, such as a T9300; 2006-2008 MacBooks (MacBook 1,1, 2,1 3,1 and 4,1); and 2008 MacBook Air (MacBookAir 1,1).
Other than one of the Penryn Macs listed above, you'll need the macOS High Sierra Patch Tool, a copy of macOS High Sierra and a USB drive with a capacity of at least 8GB.
The macOS Sierra Patch Tool is available from dosdude1.com here, and is free to download - but if you find the tool useful we would encourage you to donate to the developer.
How to install MacOS High Sierra on an old Mac: Warnings
We'd advise tech beginners against attempting this workaround: it calls for a moderate degree of tech expertise.
Remember that we're going against Apple's official advice on this one, which means that if something goes wrong - and that's always a possibility with an OS install, even if you're using official software - your warranty is unlikely to save you. Of course, it's unlikely that your 2008 Mac would be under any warranty so that may not worry you too much.
Back up your Mac before going any further. And bear in mind two more caveats.
Installing macOS High Sierra via the Patch Tool will stop Wi-Fi working if your Mac has a Broadcom BCM4321 module - which includes certain configurations of the MacPro3,1, MacBook5,2, MacBookPro4,1, iMac8,1, Macmini3,1, and MacBookAir2,1.
Dosdude1 suggests installing a new compatible WiFi card in your machine if this is the case. To find out if you have the unsupported card open the System Profiler in About This Mac > System Report and scroll down to WiFi.
At some point Apple may patch this hack and prevent it working in future. So if you're keen, and happy that the risks and difficulties are worth it for you, then jump in while you still can.
Otherwise, you might want to read How to sell an old Mac.
How to install MacOS High Sierra on an old Mac: Step by step
To do this properly you should absolutely follow the full step-by-step tutorial on DOSDude1.com. But here's a super-brief summary of what you'll be doing.
- First of all you need to format your USB drive as Mac OS Extended (Journaled) using Disk Utility.
- Open the macOS High Sierra Patch Tool, navigate to the High Sierra Installer App, select your USB drive and hit Start Operation. Boot from the USB drive.
- If you want to do a clean install, you'll need to open Disk Utility again when the installer boots, then erase the disk or partition you want to put High Sierra on - this time you have the choice of using Mac OS Extended (Journaled), or the new APFS filesystem type. (You shoudl only use APFS if your target drive is a SSD, plus if you use APFS you will not have a bootable Recovery partition - there is more detail about this on DOSDude1's page).
- Install macOS normally, then reboot back on to the installer drive. This time, open the macOS Post Install application and select the correct model of Mac. Select the appropriate volume and choose Patch, then Reboot when it's finished.
- When it reboots, your Mac should now boot into a fully working copy of macOS High Sierra.