If a disk is not booting you can do the following to see debug output during the boot process:-
1) Start the LPAR in Open Firmware
2) At the OK prompt, enter: “boot -s trap”
3) At the KDB prompt: “mw enter_dbg”
4) Next, put in: “42” – press Enter
5) Then “.” – press Enter
6) When the KDB prompt comes back: “g” press Enter
Use putty logging to capture to review later.
Thanks to IBM Tech Support for this.
