11gR2 RAC installation and the ORA-15014 error

In 11gr2, we now have to place the Voting Disk and Cluster Registry files on ASM disks and we do not have the option of placing them on OCFS or raw devices as in the case of previous versions.
When you run the root.sh script as part of the 11gR2 RAC installation,
it will create an ASM disk group CDATA and will place the OCR and Voting disk files on this ASM disk.
We provide an ASM disk discovery string where the raw devices exist on which the ASM disk groups will be built.
Initially we had provided the ASM diskstring as /dev/ocr_disk* as this was where we wanted to place the OCR and Vote files.
But when we tried to change this to create another ASM disk group, we kept getting these ORA-15031 and ORA-15014 errors.
Example:
SQL> create diskgroup DATA external redundancy disk ‘/dev/asm_disk2’ force;
create diskgroup DATA external redundancy disk ‘/dev/asm_disk2’ force
*
ERROR at line 1:
ORA-15018: diskgroup cannot be created
ORA-15031: disk specification ‘/dev/asm_disk2’ matches no disks
ORA-15014: path ‘/dev/asm_disk2′ is not in the discovery set
SQL> alter system set asm_diskstring=’/dev/asm_disk2′;
alter system set asm_diskstring=’/dev/asm_disk2’
*
ERROR at line 1:
ORA-02097: parameter cannot be modified because specified value is invalid
ORA-15014: path ‘/dev/ocr_disk1′ is not in the discovery set
SOLUTION:
SQL> alter system set asm_diskstring=’/dev/ocr_disk*’,’/dev/asm_disk*’ sid=’*’;
System altered.
SQL> create diskgroup DATA external redundancy disk ‘/dev/asm_disk2’;
Diskgroup created.