Cause: ASM was unable to find a sufficient number of disks belonging to the diskgroup to continue the operation.
Action: Check that the disks in the diskgroup are present and functioning, that the owner of the ORACLE binary has read/write permission to the disks, and that the ASM_DISKSTRING initialization parameter has been set correctly. Verify that ASM discovers the appropriate disks by querying V$ASM_DISK from the ASM instance.
SQL> alter system set asm_diskstring='' scope=spfile;
System altered.
SQL> shutdown immediate;
ORA-15100: invalid or missing diskgroup name
ASM diskgroups dismounted
ASM instance shutdown
SQL> startup
ASM instance started
...
ASM diskgroups mounted