If you expand Backplane (Channel 0), you will see the disk that has actually failed ... the red X on Backplane does not mean it is bad - it simply lets you bullet down to the bad device, so the red X doesn't mean the backplane is bad any more than the red X means the PERC 4e/Di is bad.
The fact that the replacement disk failed too could mean one of three things: 1) a bad slot, uncommon but it happens, or 2) the replacement disk is also faulty, unlikely but it also happens, or 3) the array itself is corrupt.
Right-click PERC 4e/Di and Export Log (I think that's where it is in Array Manager, which is very old software), and post it here (or somewhere, if the forum software will not take it).