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http://lime-technology.com/wiki/index.php/UnRAID_Manual#Colored_status_indicator
Colored status indicator
The significance of the color indicator at the beginning of each line is as follows:
Green: the hard drive status is Normal
Yellow: the data contents of the actual hard drive are invalid. The parity disk has this status when Parity-Sync is taking place. A data disk has this status during Reconstruction.
Red: the disk is disabled.
Blue: a new disk not currently part of the array.
Grey: indicates no disk present.
Blinking: indicates the corresponding disk has been spun-down.