MacBook Air undercalculates folder sizes on external USB drives

Why didn’t you just say, “in Get Info?”

Not sure I’ve ever used Get Info from the contextual menu.

Available and Free are different quantities. Used + Free should be the capacity. Available includes items that can be purged, so Used + Available can exceed the capacity.

Various system folders may be more than one folder combined and made to appear as one.

Without seeing the specific details, it is just a guess as to what is actually happening.

Disk Utility doesn’t show folders, just disks and categories which may have the same names as folders.

jjjefff wrote:

However, Used amount is far lower than the amount of data that I know is on this drive.

How do you 'know'? You stated that you're adding up the individual file sizes, but if the external drive is formatted as APFS, probably you don't know what you think you know.

With APFS formatting, macOS only keeps one copy of a duplicated file. Thus, you can make 10 copies of a 5 GB file, and those 10 copies will use 5 GB of your storage, not 50 GB. Moreover, if you modify one of those copies, only the changed data will be stored separately, so if you have 10 slightly different versions of that 5 GB file, maybe those 10 versions will use 5.1 GB of total space.

The bottom line is that Disk Utility is always going to give you accurate information about used and free space on the drive, it won't under- or over-calculate anything.

Where in the contextual menu do you see a folder size? I don't know of any built-in function that shows the folder size in the contextual menu.