The ORA-15260 error when attempting ot issue an alter diskgroup command is a permissions issue. It will generally be because you are logged in as SYSDBA rather than SYSASM.
Prior to 11gR2 you could log in and administer diskgroups with SYSDBA privileges but this changed with 11.2.
SQL> ALTER DISKGROUP DATA add disk ‘/dev/oracleasm/disks/DISK2’;
ALTER DISKGROUP DATA add disk ‘/dev/oracleasm/disks/DISK2’
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15260: permission denied on ASM disk group