Macbook Air eating disk space like crazy

1. Spotlight indexes your hard drive and its data is used to indicate how much on the drive is made up of Documents, Music, System etc. Unfortunately the Spotlight index is easily damaged which results in files being mis-attributed. So while Spotlight will properly indicate you have used 80GB of drive space, for example, when it reports that 60GB is Other and 20GB is System, that is likely to be wrong. See below for tips to free up space
2. CleanMyMac and other software that claim to speed up your computer and create more drive space at best are a waste of money and are known to delete things your computer needs. One thing CMM does is delete cache files. Having done that your drive appears to suddenly have 20GB of extra space. But cache files are necessary and when CMM deletes them, your computer will simply recreate them. Boom, space no longer gone.

Free Up Space: Cache files do sometimes need to be trimmed or deleted (especially after a program or system crash) but your computer will generally handle that on its own. But sometimes that doesn’t happen. Once a month I restart my computer in Safe Mode (hold the shift key while restarting) because one of its tasks is to delete & trim cache files that need it. So give that a try. Look for programs you never use and delete them - but just the programs you’ve installed. Never delete a program provided by Apple; you might not use them but they are necessary. Document files are generally small and you’ll need to delete a lot to free up space. I could actually delete all my curriculum files - presentations, lessons, the lot - and still free up less space than one movie file takes up. Media files - video, music, and digital photos - take up the most space and are what you should look to transfer to an external hard drive.

Final note: while the cloud (iCloud, DropBox, etc) are handy I’d never trust the cloud to keep my documents safe. The safest place for important documents are on 3 hard drives: your computer’s drive or external data drive, an external backup on your desk, and a second external drive that you keep outside the house. A backup is no good if it burns with the house along with your computer.

Seeing the available space disappear quickly indicates 1 of 2 things: 1) System and/or application cache files needed more space and you just provided that space. 2) you have a cache file or a system log file that is damaged and simply keeps growing. If this is the problem, restarting in Safe Mode may fix and and restore some empty space. To restart in Safe Mode, restart your computer and hold the shift key until the Apple icon appears.

If your issue is the first one (your computer really is overfull) the best thing is a tool that is independent of Spotlight. OmniDiskSweeper is a freeware tool for this and DaisyDisk is my favorite shareware tool. These 2 tools will help you figure out what is taking up all the space.

One final thought if you use TimeMachine to back up your computer. Since notebooks are often disconnected from TM, the system will safe snapshots to your computer’s hard drive instead. These should be deleted on a rolling 24 hour basis (in other words, when a snapshot is 24 hours old, it should be deleted) and all of them should be deleted when you connect back to TM and it is backed up. So if you haven’t backed up in a day or more, you have a day of snapshots taking up space. See this Apple article on how to delete snapshots. You can also delete them manually from the terminal if TM is acting up. See this Macworld article.

CleanMyMac and Sophos are some of the worst software you can install on a Mac. They are known to cause problems on Macs. I suggest you uninstall them using the uninstall procedures provided by the developers.

Keep Activity Monitor open and running so you can see if and when one of these dumps begin and what is causing them.

You could boot into Safe Mode for some deep cleaning of system caches to see if that will help.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing.

It certainly can't hurt.

Consider yourself lucky. It has been my experience when called in to help with performance problem to uninstall any such software first and the performance problems disappear.