Diskutil can't add drive to mirror

bash-3.2# diskutil appleraid add member disk3  disk8
Started RAID operation on disk8 LookingGlass
Unmounting disk
Repartitioning disk3 so it can be in a RAID set
Unmounting disk
Can't write the RAID partition map on disk3
Error: -69877: Couldn't open device

I have done this with disk8, the RAID device unmounted.

diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *240.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS SSD_System              239.2 GB   disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI                         209.7 MB   disk1s1
   2:                  Apple_HFS SysBackup               240.0 GB   disk1s2
   3:                  Apple_HFS OldUsers                759.7 GB   disk1s3
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:                 Apple_RAID                         499.8 GB   disk2s2
   3:                 Apple_Boot Boot OSX                134.2 MB   disk2s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk3
/dev/disk4
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk4
   1:                        EFI                         209.7 MB   disk4s1
   2:                  Apple_HFS Users                   3.0 TB     disk4s2
/dev/disk5
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *500.1 GB   disk5
   1:               Windows_NTFS                         134.2 GB   disk5s1
   2:               Windows_NTFS Just Stuff              365.9 GB   disk5s2
/dev/disk6
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *8.0 GB     disk6
   1:                        EFI                         209.7 MB   disk6s1
   2:                  Apple_HFS HP-8                    7.7 GB     disk6s2
/dev/disk7
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk7
   1:                        EFI                         209.7 MB   disk7s1
   2:                  Apple_HFS Recover                 499.8 GB   disk7s2
/dev/disk8
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS LookingGlass           *499.8 GB   disk8

And

bash-3.2# diskutil appleraid list
AppleRAID sets (1 found)
===============================================================================
Name:                 LookingGlass
Unique ID:            B4A0692F-7446-48F7-AF88-56AAC47B6F43
Type:                 Mirror
Status:               Online
Size:                 499.8 GB (499762790400 Bytes)
Rebuild:              manual
Device Node:          disk8
-------------------------------------------------------------------------------
#  DevNode   UUID                                  Status     Size
-------------------------------------------------------------------------------
0  disk2s2   57C06EB1-0FC6-40D8-A194-E0FEADD9DE4A  Online     499762790400

What am I missing?

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