Why An 8TB Drive Isn’t 8 Usable TB

I bought an 8TB external SSD to use as a live boot Linux device. I knew that this issue was due to the way that bits are aggregated into terabytes, but — thanks to our mate ChatGPT — here’s a nice explanation of why my 8TB SSD only shows 7,452.02 GB (7.452TB) of usable space.

The discrepancy between the advertised capacity of your SSD (8 TB) and the actual usable capacity (7,452.02 GB) is due to differences in how storage manufacturers and operating systems calculate disk space. Here’s a detailed explanation:

1. Manufacturer vs. Operating System Measurements

Manufacturer Calculation (Decimal - Base 10):
- Storage manufacturers use the decimal system, where:
  - 1 TB = 1,000,000,000,000 bytes.
  - 8 TB = 8,000,000,000,000 bytes.
Operating System Calculation (Binary - Base 2):
- Operating systems (like Windows, Linux, macOS) use the binary system, where:
  - 1 GiB = 1,073,741,824 bytes.
  - 1 TiB = 1,099,511,627,776 bytes.

When your OS displays capacity:

8,000,000,000,000 bytes ÷ 1,073,741,824 bytes per GiB = ~7,452 GiB.
Similarly:
- 8,000,000,000,000 bytes ÷ 1,099,511,627,776 bytes per TiB = ~7.28 TiB.

2. Why This Happens

Marketing: Manufacturers prefer the larger-looking decimal numbers.
Reality Check: Computers deal in binary, so the OS displays the actual capacity based on base-2 calculations.

3. Example of Conversion

If you calculate the conversion:

Advertised (Decimal): 8 TB = 8,000,000,000,000 bytes.
Actual (Binary): 8,000,000,000,000 bytes ÷ 1,073,741,824 (GiB) = ~7,452 GiB.

4. Formatting Overhead

Additionally, after formatting the drive:

Some space is used for file system structures and metadata (e.g., NTFS, exFAT, ext4), reducing the usable capacity further.

5. Practical Impact