Virtualization and Cloud Computing

I want to change my virtual box disk to immutable. When I try to do that I get the following error.

Error changing medium type from Normal to Immutable.
Cannot change the type of medium '/root/VirtualBox VMs/fedora13/fedora13.vmdk' because it is attached to 1 virtual machines
Please help me how to resolve this. My host system is RHEL 5.6

Open the Virtual Media Manager, "release" the affected image and try again.

Code:

The Virtual Media Manager allows you to

    remove an image from the registry (and optionally delete the image file when doing so);

    "release" an image, that is, detach it from a virtual machine if it is currently attached to one as a virtual hard disk.

Source: https://www.virtualbox.org/manual/ch05.html#vdis

Thanks a lot.....Issue resolved