Disk space numbers don't add up

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I’ve got a 4 TB My Cloud Home with a capacity of 3.5 TB.

I have 3 folders whose disk space collectively adds up to 3.162 TB.

But the free available space is only 3.5 GB instead of the 338 GB you would expect (3.5 - 3.162).

What is taking up all the extra disk space? How do I free it up?