IBM BladeCenter S Problem Determination And Service Manual page 22

Media tray
The media tray contains the DVD drive (which can be either a CD-RW/DVD-ROM
drive or a DVD/RW drive) and two USB v2.0 ports, which are shared by the blade
servers. The media tray also contains two battery backup units, which provide
backup for SAS RAID controller modules cache.
Note: The port on the side of the media tray is reserved for future use.
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BladeCenter S Type 7779/8886: Problem Determination and Service Guide
Note: When power is restored to the BladeCenter S chassis after a
complete loss of power and you have implemented the hard disk drives as
a mirrored array, the fault light will blink as the hard disk drive is being
resynchronized.
This resynchronization occurs because the blade servers are attempting to
power on before all of the storage modules and SAS connectivity modules
are powered up and available. The mirrored array is accessible during
resynchronization although performance may be slightly slower.
Solid when there is a drive failure.