Raid set add member: com.apple.StorageKit error 118
I know this is quite late to answer but I came across the same error today while trying to add a new member to a failed RAID1 volume.
The issue is, Disk Utility doesn't tell you the whole story.
Running the same operation through the terminal will give you the correct error message when executing diskutil appleRAID add member diskX /Volumes/RaidVolumeName:
Error: -69797: The RAID set cannot be modified because it is currently rebuilding
To avoid getting this error, you need to disable autorebuild on the RAID disk:
diskutil appleRAID update AutoRebuild 0 diskX (diskX is the virtual raid disk)
Then reboot.
After you reboot, it won't be rebuilding anymore and you can add the member, either via Terminal, or Disk Utility.
HTH others.