The diagnostics on the Dashboard of my Duo says that one of the drives is failing, although the storage tab shows the condition is good.
I have been advised to replace the drive with the same model, which apparently are not available for retail sale in Australia.
Can I replace the drive with another (red, black?) of the same capacity?
If possible, try to get a similar drive with the same specifications as the one that you had.
See if the following link helps: