My storage is full of nothing and is fluctuating. What is happening?

Your accounting of space has omitted tons of invisible files, the vm storage file that is equal to the amount of installed RAM, and the remains of OS X. The amount of free space that you do have should be at least 20GBs. It will vary as the OS writes temporary data to the disk that later is deleted by the automated system cleaning routines.

How to Free Up Space on The Hard Drive

1. You can remove data from your Home folder except for the /Home/Library/ folder.
2. Visit The XLab FAQs and read the FAQ on freeing up space on your hard drive.
3. Also, see Freeing space on your Mac OS X startup disk.
4. Free up storage space on your Mac.
5. See Where did my Disk Space go?.
6. Be sure to Empty the Trash to recover the space.
7. Replace the drive with a larger one. Check out OWC for drives, tutorials, and toolkits.
8. Use OmniDiskSweeper or GrandPerspective to search your drive for large files and where they are located.

Get Correct Storage Information

Do not use the information from the Storage section of the About This Mac dialog. Ignore the Storage information as it is typically wrong. To find out the correct information for any disk: Select a Desktop disk icon. Press Command-I to open the Get Info window and look at the topmost panel displayed. You will find the disk information displayed for Capacity, Available, and Used. If you have more than one disk/partition then repeat for each one on your Desktop.

The full installation of OS X plus all its underlying files including caches, preferences, agents, daemons, core components will ultimately take about 35GBs of space before any user files are added. To know what you are using just select your Home folder and Get Info.