IBM TotalStorage DS300 Best Practices Manual page 542

Figure 9-113 Selected device is not a disk device
e. Select from the list of LUNs the LUN from which the server will boot. The
logical drive WOLGA-BOOT was as first logical drive assigned to the initiators.
The DS400 will present the configured logical drives in that order to the
host as they where assigned to the initiator. Select the LUN 0 end press
Enter.
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IBM TotalStorage DS300 and DS400 Best Practices Guide