IBM TotalStorage DS300 Best Practices Manual page 138

Figure 4-66 Spares tab
12.The selected drive will be moved to the Spares tab. This will be a global
hotspare drive. See Figure 4-67.
Note: Select a global hotspare drive that has the same capacity as the
largest drives installed in the controller. Not all arrays and logical drives will
be protected when drives with a smaller size are defined as hotspare.
When small and large capacity drives are defined as global hotspares, the
largest one is always selected first to rebuild an array with a disabled drive.
array spare add
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Figure 4-67 Spares tab with spare drive
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IBM TotalStorage DS300 and DS400 Best Practices Guide