Oracle Error Message
ORA-15027: active use of diskgroup “string” precludes its dismount
Reason for the Error
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.
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.