While dropping a diskgroup, error may come like ORA-15073: diskgroup mounted by another instance.

Problem:

SQL> drop diskgroup NSMREDOA including contents;
drop diskgroup NSMREDOA including contents
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15073: diskgroup NSMREDOA is mounted by another ASM instance

Solution:
For rac with multiple node, you have to dismount the diskgroup on all the remote nodes.( excluding the local node)
Suppose for 2 node rac, i have logged to node 2 and dismounted the diskgroup and ran the drop command on node 1.

LOGIN TO NODE2 ( WITH SYSASM)

SQL> alter diskgroup NSMREDOA dismount;

Diskgroup altered.

LOGIN TO NODE 1 ( WITH SYSASM):

SQL> drop diskgroup NSMREDOA including contents;

Diskgroup dropped.

NOTE: If you try to drop the diskgroup on the same node, where you have dismounted, then below error will come.

SQL> drop diskgroup NSMREDOA including contents;
drop diskgroup NSMREDOA including contents
*
ERROR at line 1:
ORA-15039: diskgroup not dropped
ORA-15001: diskgroup “NSMREDOA” does not exist or is not mounted