Basically, the solution is to move your photos and documents to iCloud, so make sure you have enough iCloud storage. Click Manage and you can have all the options to optimize storage. If you are running on macOS Sierra or higher, you can optimize storage on Mac to free up space on the startup disk. It will show how many storage has been used in your startup disk by which kind of data, such as photos, documents, audios, backups, movies and other. Click on the Apple icon and select About this Mac. You can find the culprits by visiting About this Mac. Why is my startup disk getting almost full. How to See What's Taking up Space on Mac Startup Disk? If you want to directly go head to solutions, just skip the checking part and head to Part 4 directly. So, it’s time for you to find out what's taking up space on your startup disks and make room on the startup disk immediately. Or the Mac will be acting weirdly because there is not enough storage space, such as getting intolerably slow, apps crashing unexpectedly. When you are seeing this 'your startup disk is almost full' message, 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 has been selected on System Preferences > Startup Disk. If there are multiple icons showing up on your Mac, it means that you have multiple disks on your Mac. To figure it out on your Mac, you can first 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.

By default, it’s named 'Macintosh HD', although you can freely change it. Simply put, a startup disk on Mac is a disk with your operating system(such as macOS Mojave) on it. Too Many Junk Files on Mac, Can I Delete in One Click?.

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