Windows 7: Unable to see mapped network drive in administrator run CMD

BACKGROUND:

I have created 2 virtual machines both running windows 7 enterprise trial edition using virtualbox by oracle. The network I have selected is virtualbox-host only ethernet adapter. Both machines can see each other and I can transfer files between machines using the GUI interfaces.


PROBLEM:

On machine1 I created a folder "backup" and gave it full share permissions. I can see this folder from machine2.

In machine2 I mapped the network drive "backup" as z:

And I can access the backup folder via z: no problem

When I go into command prompt in machine2 I can see z: and can add and delete files no problem

The problem arises when I run cmd as an administrator from machine2. I can no longer see z:

Any ideas why? and how I can over come this.

Thanks in advance.