There're 2 reasons caused the
usable capacity will be lower than the listed capacity:
1.
When your USB flash drive is formatted,
some storage capacity is allocated for overhead, such as the boot data and file
system. Therefore, although this storage capacity is present on the USB Drive,
it is not available for the user.
2.
The major reason comes down to differences
in the way our operating systems (such as Windows and Mac OS) use storage
devices, and how USB flash drive manufacturers (as well as hard drive
manufacturers, and manufacturers of other storage devices) advertise the
storage capacity.
The second point
requires some more explanation. To a USB Flash drive manufacturer, 1KB =
1000 bytes
1MB = 1000KB
1GB = 1000MB
This means the 128GB
USB Flash drive you see in the image above contains 128 * 1000 * 1000 * 1000 = 128,000,000,000
bytes of space. This USB drive is hence labelled as an 128GB USB drive, which
is what you purchased.
Now the above
situation arises because RAM manufacturers and operating systems do not use the
same denominations of 1000. Instead, they choose to use denominations of 1024.
To them,
1KB = 1024 bytes
1MB = 1024KB
1GB = 1024MB
and so on. For
example, when a 128GB USB flash drive read by
the RAM manufacturers as: 128,000,000,000 / (1024*1024*1024), which around
119GB. Plus the reason 1 I mentioned above, so the usable capacity for end user
is 117GB.
