ORA-15073: diskgroup DATA is mounted by another ASM instance

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

If you are not able to drop a diskgroup, and ORA-15039 is returned along with ORA-15039: diskgroup not dropped, it means the diskgroup needs to be dismounted on all instances except current instance where you are issuing DROP DISKGROUP command.