Thanks for the response batty! When I run the command "esxcli storage vmfs snapshot list" it returns nothing (there is no snapshots).. Here is a snippet from our main screen under "virtual machines":

As you can see it "refers" to a volume "60226260-17ea51ee-b343-004e01a2075a" which are unavailable. 

Here is a snipped of our devices under storage (it is missing the 520GB disk):

Any new advices perhaps? I am open for a remote session if that would be easier for you?