Before doing anything else try and make sure you have a current backup.
Have a replacement disk handy. Warn everyone to save their data and to disconnect from any mounted server shares. Shut down the server remove the failing disk and install the replacement disk. Reboot the Server. Launch disk utility and monitor the rebuild of the mirror. If it's not rebuilding click the option to do so.
Depending on how much data you have it can take some time. You should be OK to use the Server whilst its rebuilding but bear in mind performance will suffer in some way whilst its rebuilding.
Currently Being ModeratedNov 21, 2012 1:27 AM (in response to Antonio Rocco)
Firstly thank you for you advice, I will be buying a new drive today, do you need to format it or does that happen when you install it?
I assume that both drives need to be the same size?
It should be exactly the same size but it's OK if it's slightly larger. You will lose the extra capacity though. For example the drives you currently have are 232.8GB in size. The replacement drive must be at least that size or larger. I would format it first, remove the one that's failed from the presented list in DU and then add the replacement by dragging its icon into the window.
It should all become obvious to you once you start the process.