ORA-15067 - When drop disks in ASM DiskGroup Normal redundancy

I have ASM Diskgroup and redundancy type is Normal ans tested to drop all disks from one Cell Server:

SQL> select NAME, TOTAL_MB, FREE_MB, STATE, TYPE, REQUIRED_MIRROR_FREE_MB, USABLE_FILE_MB from v$asm_diskgroup where name='TEST_DG';

NAME TOTAL_MB FREE_MB STATE TYPE REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
---------------------------------------- ---------- ---------- ----------- ------ ----------------------- --------------
TEST_DG 32768 18008 MOUNTED NORMAL 2978 7515

SQL> select path, header_status, os_mb,free_mb, failgroup from v$asm_disk where group_number = (select group_number from v$asm_diskgroup where name='TEST_DG') order by failgroup;

PATH HEADER_STATU OS_MB FREE_MB FAILGROUP
---------------------------------------- ------------ ---------- ---------- ----------
o/192.168.99.5/TEST_DG_FD_03_EXACEL01 MEMBER 1024 564 EXACEL01
.
.
o/192.168.99.6/TEST_DG_FD_12_EXACEL02 MEMBER 1024 552 EXACEL02
.
.

SQL> select * from gv$asm_operation;

no rows selected

SQL> alter diskgroup TEST_DG drop disks in failgroup EXACEL01 rebalance power 11 NOWAIT;
alter diskgroup TEST_DG drop disks in failgroup EXACEL01 rebalance power 11 NOWAIT
*
ERROR at line 1:
ORA-15067: command or option incompatible with diskgroup redundancy

Found ORA-15067 and found out it.

ORA-15067: command or option incompatible with diskgroup redundancy
Cause: An attempt was made to use a feature which is not permitted by the diskgroup's redundancy policy. Common examples are forcibly dropping a disk from an EXTERNAL REDUNDANCY diskgroup, using the FAILGROUP clauses with an EXTERNAL REDUNDANCY diskgroup, or using invalid template attributes.
Action: Omit the option from the command

Not sure about error and idea to test then. So, I just added new ASM Disk and tried to drop again.

SQL> alter diskgroup TEST_DG add disk 'o/*/TEST_DG*EXACEL03' rebalance power 11 NOWAIT;

Diskgroup altered.

SQL> select * from gv$asm_operation;

INST_ID GROUP_NUMBER OPERA STAT POWER ACTUAL SOFAR EST_WORK EST_RATE EST_MINUTES ERROR_CODE
---------- ------------ ----- ---- ---------- ---------- ---------- ---------- ---------- ----------- -----------
1 4 REBAL RUN 11 11 1931 2051 8646 0

SQL> select * from gv$asm_operation;

no rows selected

SQL> select NAME, TOTAL_MB, FREE_MB, STATE, TYPE, REQUIRED_MIRROR_FREE_MB, USABLE_FILE_MB from v$asm_diskgroup where name='TEST_DG';

NAME TOTAL_MB FREE_MB STATE TYPE REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
---------------------------------------- ---------- ---------- ----------- ------ ----------------------- --------------
TEST_DG 49152 34184 MOUNTED NORMAL 5004 14590

SQL> select path, header_status, os_mb,free_mb, failgroup from v$asm_disk where group_number = (select group_number from v$asm_diskgroup where name='TEST_DG') order by failgroup;

PATH HEADER_STATU OS_MB FREE_MB FAILGROUP
---------------------------------------- ------------ ---------- ---------- ----------
o/192.168.99.5/TEST_DG_FD_14_EXACEL01 MEMBER 1024 696 EXACEL01
.
.
o/192.168.99.6/TEST_DG_FD_12_EXACEL02 MEMBER 1024 708 EXACEL02
.
.
o/192.168.99.7/TEST_DG_FD_10_EXACEL03 MEMBER 1024 712 EXACEL03
.
.

SQL> alter diskgroup TEST_DG drop disks in failgroup EXACEL01 rebalance power 11 NOWAIT;

Diskgroup altered.

SQL> select * from gv$asm_operation;

no rows selected

SQL> select path, header_status, os_mb,free_mb, failgroup from v$asm_disk where group_number = (select group_number from v$asm_diskgroup where name='TEST_DG') order by failgroup;

PATH HEADER_STATU OS_MB FREE_MB FAILGROUP
---------------------------------------- ------------ ---------- ---------- ----------
o/192.168.99.6/TEST_DG_FD_12_EXACEL02 MEMBER 1024 616 EXACEL02
.
.
o/192.168.99.7/TEST_DG_FD_09_EXACEL03 MEMBER 1024 584 EXACEL03
.
.

SQL> select path, header_status, os_mb,free_mb, failgroup, mount_status from v$asm_disk where path like '%TEST_DG%' and failgroup='EXACEL01';

PATH HEADER_STATU OS_MB FREE_MB FAILGROUP MOUNT_S
---------------------------------------- ------------ ---------- ---------- ---------- -------
o/192.168.99.5/TEST_DG_FD_10_EXACEL01 FORMER 1024 0 EXACEL01 CLOSED
.
.
.

MOUNT_STATUS should not be “CACHED” for DROPPED Disks.

I dropped all Disks from one cell server. I'm curious. My ASM DiskGroup is Normal redundancy type, could not drop!!! had to add disks and tired again.