How do I save MacBook Air data to an external hard drive

The best and easiest way to transfer your data from an old to a new is to connect the Time Machine backup drive of the old Mac to the new Mac before you start up the new one for the first time to set it up. At a point in the setup process you will be asked if you want to transfer data from another computer, and you will answer yes and follow the steps presented.

❝Use Migration Assistant

Migration Assistant is included with your Mac—both as an app and as part of the setup assistant that opens when you first start up a new Mac. 

On your new Mac

1. Open Migration Assistant:

• • If your Mac starts up to a setup assistant, which asks for details like your country and network, skip to the next step, because migration is part of that setup. 
  • If you already completed initial setup of your new Mac, open Migration Assistant. You can find it in the Utilities folder of your Applications folder. Or use Spotlight to find and open it. If asked for permission to make changes, enter your administrator password.

1. When asked how you want to transfer your information, select the option to transfer from a Mac, Time Machine backup, or startup disk. Click Continue.❞

Transfer to a new Mac with Migration Assistant - Apple Support

MA will not transfer operating system data and other applications if you don't wish it to.

If you have already setup your new Mac, you can still transfer you old data f by using Migration Assistant at any time.