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 next update, macOS 10.13, will be unveiled at WWDC 2017 in June. (Here's how to get WWDC tickets.)
The yearly macOS updates are free to install from the Mac App Store, 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 certain models will for the first time be deemed too old and underpowered to make the grade. Apple advises that macOS Sierra, for instance, 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 not everything is lost. A patch tool written by a developer known as DOSDude1 enables you to install macOS 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.
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 Sierra Patch Tool, a copy of macOS Sierra and a USB drive with a capacity of at least 8GB.
The macOS Sierra Patch Tool is available from dosdude1.com, 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 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, very few 2008 Macs are still under 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 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.
And at some point Apple is likely to 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.
Read next: How to sell an old Mac
How to install MacOS 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 Sierra Patch Tool, navigate to the 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 Sierra on - again, use Mac OS Extended (Journaled).
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.