Oracleasm deletedisk failed Unable to clear disk

Question

I have dropped the disk from ASM diskgroup:

> ALTER DISKGROUP DATA01 DROP DISK DISK1;

Oracle did a rebalance operation and then dropped the disk.

Now, I want to delete the disk from ASM library and return the disk to the system:

# oracleasm deletedisk -v DISK1
  Clearing disk header: oracleasm-write-label: Unable to open device "/dev/oracleasm/disks/DISK1": 
  Device or resource busy failed
  Unable to clear disk "DISK1"

The system details: 11.2.0.1 2-node RAC

Why does ASM not clearing the disk header? Where to see the detailed log?

------------------------------------Edit1--------------------------------------------

# fuser /dev/oracleasm/disks/DISK1

returns nothing on both nodes