ORA-15039 and ORA-15001 while Dropping an ASM disk group

I was trying to drop a diskgroup and failed with ORA-15039 and ORA-15000. Following is what was returned.

SQL> drop diskgroup oradata; drop diskgroup oradata * ERROR at line 1: ORA-15039: diskgroup not dropped ORA-15001: diskgroup "ORADATA" does not exist or is not mounted

ORA-15032 ORA-15040 ORA-15042 while mounting a diskgroup

 You may face these 3 error messages together if you are trying to mount an ASM diskgroup and one or more disks of this diskgroup are missing. In case of an external redundancy diskgroup, you need at least one ASM disk to be available to mount a diskgroup. For a normal redundancy diskgroup, you need at least 2 disks for the ASM diskgroup before it can be mounted. And in case of a high redundancy diskgroup, you need at least 3 disks to mount a diskgroup. After an ASM instance restart, an ASM diskgroup may not get mounted automatically if it is not fulfilling the requirements explained above.