6800 server can NOT boot from second disk

[mcclnx mcc]

We have  DELL 6800 server  with Redhat AS 4.3
installed.  The disk  configuration are:

Virtual disk 0  (/dev/sda)  0:0 and 0:1  (mirror)
Virtual disk 1  (/dev/sdb)  1:0 and 1:1  (mirror)
Virtual disk 2  (/dev/sdc)  1:2 and 1:3  (mirror)

The system normally boot from  /dev/sda0.  I am doing
recovery test and want boot from /dev/sdc.  I use
"mkbootdisk" make a bootable CD.

The procedures I am doing to boot from  Third disk 
are:

1. rebbot

2. ctl-M to PERC 4e/dc  firmware  menu.  Cahnge boot
disk to "2"

3. put bootable CD to CD-ROM and reboot.

It still boot to first disk.  I found  it did NOT
chnge third disk  (/dev/sdc)  to /dev/sda.

The procedures are correct on DELL 6850 (it will
change third disk to /dev/sda).

Can anyone tell me how to do it ot how to boot from
third disk?

Thanks.

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