Recently we faced this error while un-mounting /u01 file system.
You can get this error in two case either some one is logged in into the file system or some process is using this file system.
We have checked that no one is using /u01 file system now only one option was there , some process is using /u01 file system which is not allowing us to un-mount it.
We can check with below command that which processes are associated with /u01 file system.
This command will show the list of process ID with user which are using /u01 file system.
Now kill both the process and unmount the file system.