ReadyNAS 104 Disk Failure

I have what I think is a drive failing, but I cannot locate in the manual what I need to do to replace this drive?

It seems to be Disk 2 but I cannot tell which phsyical drive this is, as in do the numbers start from the right or the left and also do they start Disk 0 to Disk 3 or do they start Disk 1 to Disk 4?

I have attached an image of the Log file, only found this when trying to update the firmware, had not received any e-mails so first that I had heard about it.

Also what is the proceedure to change this disk? again cannot seem to locate a guide to replacing a drive that is faulty, can I do this while the ReadyNAS unit is switched on and working, or do I shut the unit down and replace the drive when the unit is switched off?

Again unsure as to what drive I need to pull from the unit

Any help would be much appreciated...

James