Unless you have made some changes to your computer, this should be the hard drive that was in the laptop when you first bought it. The startup disk on your MacBook Air is the hard drive that is storing all of your operating system files. The best way to understand the problem that you are having is to identify the actual issue.

What is the Startup Disk on a MacBook Air? In that case, you are going to need to clean the full startup disk on your MacBook Air by deleting some of the files that you aren’t using, or don’t need. You can mitigate this problem by saving files to a cloud storage platform or an external hard drive, but you might find those options either impractical or cumbersome. The 128 and 256 GB versions of the Air are really easy to max out, and a full hard drive on my MacBook Air is something that I have been living with since only a short while after I started using one. The full startup disk error on the MacBook Air is something that you will probably encounter at some point as someone that owns or works with this laptop.

How to clear space on macbook air