There's a terrific, free application called OmniDisk Sweeper that can help you discover what files and folders are occupying the majority of space on your drive. You can run it simply by double-clicking to launch, but you may not get the whole story as it won't show you the size of files you don't have permission to see (for example system files or other users' files). 


If you run OmniDisk Sweeper just by double-clicking it and it doesn't seem to show all the space, you can also run in a special mode that shows you all files and folders owned by every user on the machine. Note that this can be a dangerous undertaking as it also allows you to delete anything on your drive--including items critical to the operation of the computer. As such, be very careful when deleting any files or folders. If you don't know what something is, don't just delete it and hope for the best as the worst will likely ensue! With that very important caveat in place, here are the instructions.


If you are a Bionic Cat monitoring client:


  • Click on the Bionic Cat icon in your menubar
  • Choose "Install Software"
  • Select the "Applications" icon at the top
  • Click install by the proper version of OmniDisk Sweeper. (1.10 is for Sierra and High Sierra; 1.9 is for OS versions prior.)
  • OmniDiskSweeper should launch
  • Select the drive in question and click “sweep” and give it time to complete. This will show you where everything on your drive is


If you are not a Bionic Cat monitoring client, why not?! Here are the manual instructions:


  • Head to https://www.omnigroup.com/more to download OmniDiskSweeper
  • Once downloaded, open the DMG and add OmniDiskSweeper to your Applications folder
  • Open the Terminal program from the Applications >> Utilities folder
  • Paste the following into the terminal and hit enter:
  • sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper
  • Type your computer password when prompted and hit enter (you won’t see anything show up as you type; this is normal)
  • OmniDiskSweeper should launch
  • Select the drive in question and click “sweep” and give it time to complete. This will show you where everything on your drive is


Again: please, please, please don't delete anything you're unfamiliar with. Ask beforehand if uncertain. Delete responsibly. :-)