While dismounting a diskgroup, got the below error:

SQL> alter diskgroup NEWTST dismount;
alter diskgroup NEWTST dismount
*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15027: active use of diskgroup “NEWTST” precludes its dismount

Solution:

There might be some running databases using that diskgroup.

check v$asm_client,for active connections to that diskgroup

set pagesize 299
set lines 2999
col instance_name for a21 
select a.instance_name,a.db_name,a.status from v$asm_client a, v$asm_diskgroup b 
where a.group_number=b.group_number and b.name='NEWTST';

INSTANCE_NAME         DB_NAME  STATUS
--------------------- -------- ------------
TESTDB21              TESTDB2  CONNECTED
+ASM1                 +ASM     CONNECTED

The database TESTDB2 is running and using this diskgroup, shutdown the database and try to dismount.

srvctl stop database -d TESTDB2

SQL> alter diskgroup NEWTST dismount;

Diskgroup altered.