IBM 8676 - Eserver xSeries 335 Installation Manual page 38

Hard disk drive status LEDs: When these LEDs are lit, it indicates that the
associated SCSI hard disk drive has failed. If an optional RAID adapter is installed
in the server and the LED flashes slowly (one flash per second), the drive is being
rebuilt. If the LED flashes rapidly (three flashes per second), the controller is
identifying the drive.
Diskette drive activity LED: When this LED is lit, it indicates that the diskette drive
is in use.
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IBM xSeries 335 Type 8676: Installation Guide