I’ve been having some strange issues lately with the 256GB SSD on my M4 MacBook. The storage numbers just don’t seem to add up — the amount of space used plus the available space doesn’t equal the full 256GB capacity, even after checking the “hidden” files.
What’s even more confusing is that storage space seems to disappear by itself when I’m not doing anything on the computer. For example, overnight I can lose around 6GB of free space with nothing changed.
Has anyone else experienced this issue or have any ideas what could be causing it?
Your case sounds very similar to one I experienced before. In my situation, the culprit turned out to be the local Time Machine snapshots stored on my SSD, which caused my storage usage to not add up correctly.
I opened Terminal and ran diskutil list / to check the local SSD. The output showed that the Data volume was much larger than expected, even though the size of my user directory was nowhere near that total. That led me to realize the extra space was being consumed by the local Time Machine snapshots.
Yes, the local Time Machine snapshots are whoppingly large in size, eating more space than expected. Cuz it backed up all the file data. I recommend you try the File Keeper feature in iBoysoft DiskGeeker for Mac. It can back up your Mac volumes in snapshots only with MBs in size, cuz it only saves the file data storage path, not all the specific file data themselves. I bet that’s what you need for files backup.
Storage reporting for APFS volumes in macOS—especially the boot volume—is more complicated than it appears. As others have pointed out, the Free storage value shown in Disk Utility (or Apple System Information) is the only truly accurate measure of unused disk space.
The Available storage value displayed throughout macOS can be misleading because it is not the same as Free space. Likewise, the Used storage value may not always be accurate due to the way APFS manages storage. In some cases, it can even report more space in use than the drive’s physical capacity or display a negative value.
Even if you manually total the sizes of every visible file and folder, you may still be unable to account for all the storage on the drive.
For example, on one of my Macs, there are still about 20-40GB of unaccounted-for space. Even with Full Disk Access and root/administrator privileges, some areas of the boot volume remain inaccessible or hidden from users.