Firstly, reactivate the underlying volume. Do the instructions below if the precious action is not helpful. If the disk is in the Online status, try to reactivate it again by yourself. If the disk is in Offline or Missing state, there is a possibility that the disk is nonfunctioning and the connectors are loss or failed. If the nonfunctioning is the culprit behind Failed Redundancy, please follow what I have mentioned in Repair Damaged Hard Disk to see whether there is a chance to bring the drive alive. If there is no any chance, try to recover data from the disk with MiniTool Partition Wizard. To replace the failed mirror, right-click any of disk areas in Disk Management, select the Remove Mirror option and then follow the on-screen instructions. To replace the disk, you should import a healthy disk and then convert it to dynamic with some utilities. Then do what you want.