Thanks for the information, that's very helpful.
On a whim I checked the local BestBuy and they had an 8TB WD Red SAS drive which I purchased.
So from what I have read given my Parity drives are 6TB the max size of this new drive in the array will be 6TB.
In the meantime I will shutdown and restart the server and see if the 'failed' drive comes back online.
If so I will download a new diagnostics and post.
Again, thanks very much for the quick response, very much appreciated.
Cheers,
Sparkie