Why Can't I Get Full Data Space on Storage Devices?

Understanding Decimal and Binary Systems

When discussing storage capacity issues, it's essential to understand the differences between decimal and binary systems. Most popular programs and operating systems, including Microsoft Windows (BIOS, FDISK) and older versions of MacOS, use the binary system, where the smallest unit of storage is a bit. Eight bits equal one byte, and one kilobyte (KB) is 1,000 bytes. However, when it comes to measuring storage, manufacturers and operating systems often employ a disparity between these systems.

The Discrepancy Between Base 10 and Base 2

In the metric system, which is used to measure storage (Kilo, Mega, Giga, etc.), the prefixes are based on powers of ten, such as Kilo (1,000), Mega (1,000,000), and Giga (1,000,000,000). These prefixes don't exist in the binary system, which uses powers of two, so different prefixes like Kibi, Mebi, and Gibi are often used. This discrepancy is why when a storage device is plugged into a Windows PC, the total available space is slightly less than advertised.

How Manufacturers Display Storage Capacity

Manufacturers tend to display storage capacity using base 10 for human understanding. For example, a 1 GB drive is often advertised as having 1,000,000,000 bytes. However, modern operating systems, including Windows and macOS, calculate storage capacity using base 2, where 1 MB is actually 1,048,576 bytes. This results in a difference where the advertised capacity is slightly larger than the actual usable capacity.

Why Operating Systems Lie About File Sizes

Your operating system might be misinforming you when displaying file sizes. The MB used by operating systems often refers to MiB in the binary system, which is closer to 1,048,576 bytes rather than 1,000,000 bytes. Similarly, TB displayed might actually be TiB, where 1 TiB 1,099,511,627,776 bytes. This slight difference means that the full advertised capacity of a 1 TB drive is rarely fully utilized by the operating system.

Further Factors Affecting Storage Capacity

Additional factors can further reduce the usable storage capacity of a drive. These include overhead factors such as the space needed for metadata, folder and file names, and the use of file system structures like clusters and sectors. Files smaller than these groups may still take up a full cluster, wasting space.

In conclusion, the difference between the advertised capacity and the actual usable capacity is due to the discrepancy between base 10 and base 2 systems, as well as the way operating systems handle and display file sizes. Understanding these differences can help you better estimate the actual usable storage space on your devices.