So find out what's taking up space on your startup disks and make room on startup disk immediately. Or the Mac will be acting weirdly because there is no enough storage space, such as getting intolerably slow, apps crashing unexpectedly. When you are seeing this 'your startup disk is almost full' messages, it means that your MacBook or iMac is running on low space and you should clear your startup disk as soon as possible.

What Does It Mean When Your Startup Disk Is Full? However, you only need to clean the startup disk that your Mac is currently running on, which is the one that have been selected on System Preferences > Startup Disk.

If there are multiple icons showing up on Mac, it means that you have multiple disks on your Mac. Just to be sure, make all disks show up on your desktop: click Finder on the Dock, select Preferences and check 'Hard disks'. Usually there is only one startup disk on a Mac, but it is also possible that you have divided your hard drive into different disks and get multiple startup disks. Simply put, a startup disk on Mac is a disk with a operating system(such as macOS Mojave) on it. How to Clean Startup Disk on MacBook/iMac/Mac Mini?.How to See What is Taking up Space on Mac Startup Disk?.What Does It Mean When Your Startup Disk Is Full?.

Mac disk cleaner and speed up computer