Hard Disk Capacity

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Understanding Hard Disk Drive Capacity
Hard Disk Drive (HDD) manufacturers measure storage capacity using the decimal system. For example, 1GB (GigaByte) is calculated as exactly 1,000,000,000 bytes.
Windows and most operating systems use the binary system of measurement. In binary, 1GB is calculated as 1,073,741,824 bytes. The difference in how the decimal and binary numeral systems measure a GB is what causes a 120 GB Hard Disk Drive to appear as approximately 112 GB in windows. Even though the storage drive still has 120 billion bytes. Either way the storage capacity is the same no matter which system of measurement is used. No capacity is missing, it’s just calculated differently. In windows, 1024 MB = 1GB.

Partitions

Partitions are the practice of dividing storage space from a drive into separate areas / allocations. Each partition can have it’s own drive letter, have a different format and or a different purpose.
Example: laptops typically now come with a restore partition on the drive which contains the files needed to perform a system restore. As this is the same HDD in the laptop, it will also reduce the amount of space seen on your HDD within windows.