Create/Drop Diskgroup

Overview using force option with mount,dismount drop and create DG

  Mount force
  The force option becomes a must when a disk group mount reports missing disks. This is one of the cases when 
  it's safe and required to use the force option. Provided we are not missing too many disks, the mount force 
  should succeed. Basically, at least one partner disk - from every disk partnership in the disk group - must 
  be available.

  Create Diskgroup force 
  If the disk to be added to a disk group is not CANDIDATE, PROVISIONED or FORMER, I have to specify force next 
  to the disk name. This will destroy the data on that specified disk(s).

  Forcing disk group drop
  To drop a disk group I have to mount it first. If I cannot mount a disk group, but must drop it, I can use the 
  force option of the DROP DISKGROUP statement, like this:
  SQL> drop diskgroup PLAY force including contents;

  Forcing disk group dismount
  ASM does not allow a disk group to be dismounted if it's still being accessed. But I can force the disk group 
  dismount even if some files in the disk group are open. Here is an example:
  SQL> alter diskgroup PLAY dismount;
  alter diskgroup PLAY dismount
  *
  ERROR at line 1:
  ORA-15032: not all alterations performed
  ORA-15027: active use of diskgroup "PLAY" precludes its dismount

  Note that the forced disk group dismount will cause all datafiles in that database to go offline, which means they 
  will need recovery (and restore if I drop disk group PLAY)

  For details please read

•  http://asmsupportguy.blogspot.co.uk/2011/12/forcing-issue.html