Mac storage basics · External storage and adding space
Why does my new external drive show less space than the box says?
Short answer: Mac shows decimal units like the box, so a 1 TB drive shows close to 1 TB minus format overhead. Much less suggests binary units in another tool or an extra partition.
On a Mac, a 1 TB drive should show close to 1 TB, because macOS and drive makers both count in decimal units. A little is used by the format's own structures and, on some drives, hidden partitions.
If you see a much smaller figure, such as 931 GB for a 1 TB drive, you're probably looking at a Windows tool or a utility that counts in binary units. A drive showing far less than either figure may have an extra partition — check Disk Utility → View → Show All Devices.