Cause: An ALTER DISKGROUP ... DISMOUNT command specified a diskgroup which had clients with open files in the diskgroup. Diskgroups cannot be dismounted until all open files in the diskgroup are closed. See the solution below.
Error solution: Stop all clients that are using this diskgroup and retry the ALTER DISKGROUP ... DISMOUNT command. Query the V$ASM_CLIENT fixed view in an ASM instance or use ASMCMD LSOF to list active clients.
Database version: Oracle 11g Release 2 (11.2)