How can I recover from a Data Volume Failed to Mount message?

test if it can be used on another PC
try with other cables
if that does not help - try running the WD diag :  http://support.wdc.com/product/download.asp?groupid=904&lang=en

Maybe the disk inside the NAS has failed. NAS usually have a web interface from which you can configure them and run tests. Use that interface to check it's status. You could also check whether a firmware upgrade could help.

If none of that helps, it is likely the NAS or disk is bad. If that is the case and you still have warranty, return it for a warranty replacement.

This is being accessed wirelessly, not connected directly to any PC.  My interest is in accessing the data, and to avoid returning it with data to the manufacturer.

Then you'd have to remove the disk from the NAS, which voids the warranty...

Reputed manufacturers like WD can be trusted to keep any data they should find confidential.

did hyou run the diag?  it can help fixing the problem

It's a NAS. You can't run diags on a NAS, unless it's built-in OS has something included for diagnosis. But that will usually just be SMART readouts you get there.

in the site i posted, you can select their product - and the diag for it

On that link there is no diagnostic for it, just a utility that helps you manage it, firmware etc.

it seems you are correct rindi  -i did not read all on that page
and assuming  is the worst habit on the internet
sorry for that

You should strongly consider copying the data off of there and reformatting the array, if not sending it back for replacement.  You're telling us that its reporting incorrect information about space; sounds like the config is damaged or corrupted.  Which means its safe to assume its unstable.

Im in the middle of a 6 drive recovery process myself and you dont wanna be here with me.  Trust me!

I wa able to see te files.