We had a problem when the OS marked our raid 5 partition as hidden and greyed out the icon. We think the cause is a full time machine, but we aren’t sure. The eventual fix for this was using a tool in XCode (a free development tool from apple) to run a command which changes the partition from hidden to visible. Once you install xcode and command line tools its a simple command:

SetFile -a v **your file or volume here**

You must have Xcode (and in preferences of xcode the cmd line tools module) installed to use this command and will have to eject and remount the drive to see the change.

In the process of finding this fix we decided to update  the firmware on our Areca-8050 external unit.

We ran into some problems in the process, which you can read about here.