Add a new virtual disk to a very critical existing VM

Hi to all,

my question is very simple. I have to add a new virtual disk to an existing Windows Server 2008 Standard virtual machine (we use vSphere 5). This machine

is very critical because it's the fileserver of the company. Is there any risk to add the new virtual disk to the VM ? Is it better to add it with the vm switched off or can i proceed also with the vm running without risk ? I tried with a test vm and everything works fine and i know to be very paranoic, but in these moment i feel always a big responsibility on me.

Thanks to all.