Lenovo IdeaPad U550 - Manual (Page 5)

In addition, if a portion of the hard disk space has been used for some
specific purpose, then the capacity indicated by the operating system will
be even less than X × 0.931 G.
II. A portion of the hard disk space is used for specific purposes
Lenovo computers may come with preinstalled OneKey Rescue System, so
the hard disk has a special portion partitioned before delivery, which is
used to store hard disk mirror and OneKey Rescue System program files.
The size of the reserved space for this partition varies according to the
model, operating system and software of the computer. For the sake of
safety, this partition is not apparent, which is commonly referred to as a
“hidden partition”.
In addition, after the hard disk is partitioned or formatted, the system will
assign a certain amount of hard disk space for the system files.
For the above reasons, the available hard disk space indicated by the
operating system is always less than the computer’s nominal hard disk
capacity.