A box says 1 TB. One utility shows about 931, while another reports a figure close to 1,000 GB. Those numbers can describe the same quantity of bytes. Before suspecting a faulty drive, read the unit and the name of the field beside it.
Count the same bytes two ways
A decimal terabyte is 1,000,000,000,000 bytes. A gibibyte, written GiB, is 1,073,741,824 bytes. Dividing the first number by the second gives about 931.3 GiB. The calculation changes the unit, not the amount of storage.
NIST documents the distinction between decimal and binary prefixes. Software does not always label its display as clearly, so a number marked GB may need checking against that tool’s explanation or its exact byte count.
Capacity and free space answer different questions
Apple also notes that formatting uses space and that the operating system can occupy more. The decimal-to-binary conversion alone therefore does not explain every difference you see.
Ask what the selected screen measures:
- The physical device: how much storage the drive exposes.
- A volume: the portion being shown as a usable disk.
- Free space: what remains after that volume’s contents and storage overhead.
For example, a disk split into two volumes should not be judged by the size of just one. Nor should the free-space figure of a laptop already holding photographs be compared directly with the capacity on its box.
Investigate a mismatch without erasing anything
Write down the model, advertised capacity, displayed units and whether the figure is total or free. Inspect the device and its volumes in the system’s disk utility without changing them. If the difference remains unexplained, send those details to the manufacturer or seller, leaving personal filenames out.
A plausible capacity number is not a complete health test. Equally, formatting a drive just to make its number match the box is not a diagnosis. Preserve the files while you work out which measurement you are comparing.