Cause:

I try to drop an asm diskgroup which is not properly created but i get the error that diskgroup is not mounted

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

Error code:

[oracle@exdbadm02 ~]$ oerr ora 15039 && oerr ora 15001
15039, 00000, "diskgroup not dropped"
// *Cause:  An attempt to drop a diskgroup failed.
// *Action: See the associated messages for details about why the drop
//          was not successful.
//
15001, 00000, "diskgroup \"%s\" does not exist or is not mounted"
// *Cause:  An operation failed because the diskgroup specified does not
//          exist or is not mounted by the current ASM instance.
// *Action: Verify that the diskgroup name used is valid, that the
//          diskgroup exists, and that the diskgroup is mounted by
//          the current ASM instance.
//

When i checked the logfile,i can see couple of errors because the physical disks are removed from the storage tier

ORA-15032: not all alterations performed
ORA-15017: diskgroup "DATA" cannot be mounted
ORA-15063: ASM discovered an insufficient number of disks for diskgroup "DATA"
ERROR: alter diskgroup DATA MOUNT /* ASMCA */
Sat Feb 06 16:58:19 2021
NOTE: No asm libraries found in the system
ASM Health Checker found 1 new failures
Sat Feb 06 16:59:11 2021
SQL> drop diskgroup DATA
ORA-15039: diskgroup not dropped
ORA-15001: diskgroup "DATA" does not exist or is not mounted

Workaround:

If graceful drop doesnot work ,then we can forcefully drop the diskgroup with contents option

SQL>  drop diskgroup DATA force including contents;

Diskgroup dropped.

Oracle forcefully erase the diskgroup header and metadata of the diskgroups

NOTE: Assigning number (2,9) to disk (o/192.168.56.33/DATA_CD_cell19_stocell)
NOTE: Assigning number (2,8) to disk (o/192.168.56.33/DATA_CD_cell09_stocell)
NOTE: erasing header on grp 2 disk DATA_CD_CELL09_STOCELL
NOTE: erasing header on grp 2 disk DATA_CD_CELL19_STOCELL
NOTE: Disk DATA_CD_CELL09_STOCELL in mode 0x19 marked for de-assignment
NOTE: Disk DATA_CD_CELL19_STOCELL in mode 0x19 marked for de-assignment
SUCCESS: diskgroup DATA was force dropped
SUCCESS:  drop diskgroup DATA force including contents