Dell XPS 15 (L502X, Early 2011) - Manual (Page 63)

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Using Your XPS Laptop
Free Fall Sensor
Free fall sensor protects your computer hard
drive from possible damage, by detecting
a free fall state caused by accidentally
dropping the computer. When a free fall
state is detected, the hard drive is placed in
a safe state to protect against damage to the
read/write head and possible data loss. Your
hard drive returns to normal operation, when
the free fall state is no longer detected.