HOW TO Clone and boot from an external drive
How to slim down that old, small hard drive so your Mac runs faster
1 Check the drive size
If your removable internal hard drive is failing but not yet dead, the easiest way to replace it is to clone it. First, check the size of drive you need. In a notebook it’s likely a 2.5–inch drive, but a desktop might take a 3.5–inch model.
2 Download CCC
Connect your new drive to your Mac using a docking station or a USB caddy, then go to https://bombich.com and follow the steps to download a copy of Carbon Copy Cloner. It’s available for macOS 10.12 (Sierra) or later.
3 Swap the drives
Use Carbon Copy Cloner to clone your hard drive to your new drive. The trial version of the app is sufficient. When it’s done, swap your internal drive for the new one. You might want to run Disk Utility’s First Aid afterwards.
4 Make an external one
If your old Mac’s hard drive is too damaged to boot from, you can make an external boot drive instead. For this guide, we’re going to use OS X 10.7 (Lion). It’s old, but it still works on a lot of legacy Macs.
5 Download the OS
Go to https://support.apple.com and search for the Lion installer; or, you can download it from the Mac App Store. For this guide, you might need to use an older Mac if your main machine is an Apple silicon model.
6 Prep the drive
Find an external drive or a USB flash drive of at least 8GB to use for the boot drive. If there’s anything else on there you want to keep, transfer it now. Then, in Disk Utility, erase it in Mac OS Extended (Journaled) format.
7 Copy file
Ctrl–click on your "Install OS X 10.7 Lion" app and click on Show Package Contents. Next you need to navigate to Contents > SharedSupport and Ctrl–click on InstallESD.dmg. Then select Copy "InstallESD.dmg".
8 Make the boot drive
Ctrl–click on the desktop and select Paste Item. In Disk Utility, click on your external drive in the sidebar, and click the Restore tab. Click the Image button, and navigate to the InstallESD.dmg file. Click Choose, then Restore.
9 Restart your Mac
With your new boot drive plugged in, restart your Intel Mac. When you hear the startup chime, press and hold the Option key until this screen appears. Choose your boot drive with the arrow keys and press Return.
ANOTHER WAY TO make your old Mac run faster is to declutter its hard drive — getting rid of old, unwanted apps, backing up and archiving media and files you’re not likely to need any time soon, and moving currently used media to external drives. Moving large media files such as your music, movie and photos libraries does much to free up valuable hard drive space.
Moving your media is sensible for other reasons too. If the hard drive inside your Mac is as old as the Mac itself (which it probably is), it could fail at any time. Therefore moving your media to a new external drive is a sensible safeguard against failure. And don’t forget to take a cautious approach and make regular backups too.
For unwanted apps, it’s best to use a clean installer such as the free AppCleaner (https://freemacsoft.net/appcleaner), or AppZapper (https://appzapper.com), which allows five uninstalls before you have to pay. Whatever operating system you use there’s likely to be a clean installer that runs on it, so have a look around. Then delve into your Mac’s Applications folder, and drag anything you don’t need onto the clean installer window (they nearly all work like that). Uninstall when prompted, and the app is completely removed, including system files that might be left behind if you simply dragged the app into the bin and emptied it.
Moving your media to an external drive also frees up space on your boot drive. As this article is about extending the life of an old, tired Mac, we’re using an ancient MacBook Air running macOS High Sierra to show you how it’s done in the walkthrough below, but these steps should also work pretty much the same for most older operating systems.
With your photos, music and videos on an external drive, not only can you delete them from your main drive to save valuable space, but you also have a backup in case that drive should fail.