Incompatible device backing specified for device '0'

I am facing this error meesage while trying to configure a shared disk for two different guest located on two different host.

I installed windows 2008 enterprise 64 bit on both the node.

Both node are on different host. I wanted to create a failover cluster.

I assigned a RDM disk to first node and selected a datastore which is located on SAN.

Then I tried to assign this disk to second node by selecting "use an existing virtual disk". I selected the same "Disk File Path" .

But i am just seeing the error saying "Incompatible device backing specified for device '0'".

I am using ESXi 5.0 evaluation.

I tried some fixes like renaming the .vmdk files and deleting cdrom from the add hardware - but none of them worked actually.