Hitachi AMS 2100 Hardware Manual page 25

NOTE: The power ON/OFF buttons on the front bezel actuate the Main
Switch on the front of the base unit (behind the front bezel). The Main
Switch is actually a standby switch and controls power to all components in
the base unit except the power supplies. The power supplies do not have a
power switch and turn on as soon as the power cables are connected to
them. They can deliver power to the rest of the unit after successful
completion of their power on self test (POST). See Chapter 4, Hardware
components and Chapter 6, Power On/Off procedures or details.
When the power switch is toggled to the OFF position, power is still applied
to the power supplies. To completely remove the power from the unit,
remove both power cables from the unit or disconnect them from the power
source.
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Introduction
Hitachi AMS 2100/2300 Storage System Hardware Guide