You can check it by going to System Preferences > Startup Disk. This disk is named after your operating system. All your files, including the system and applications, are on it altogether. For a MacBook, usually it will have only one disk. Often times, it also has applications in it. This disk contains the Operating System for your Mac, so that it can start successfully. The name is actually pretty self-explanatory. Look no further, and we’ll start with the basic question first – what is a startup disk? If you want the solutions right away, you can also jump ahead and scroll down to the How do I Clean up Start up Disk on Mac? section. What caused this to happen? How can you solve it? What do you need to do to clear up disk space and system storage on Mac? As alarming as this alert may seem, we have some pretty solid solutions for it. And one day, you will see an alert popup window that says, “Your startup disk is almost full.” However, when you have used this laptop for a few years, the excitement is now completely gone as it performs slower and slower. We all know that feeling… receiving a brand-new MacBook… the smell of the packaging reminds us how smooth this computer will work.