How to Rebuild Software RAID 10 in Mountain Lion – Command Line

Removal

First, replace the disk 😉

Open a terminal session on affected system and run
diskutil appleRAID list
find the UUID for the affected drive and the UUID for theRAID set. Then fill in the blanks below:
sudo diskutil appleRAID remove <drive UUID> <RAID UUID>
For example:
sudo diskutil appleRAID remove EEDD0AD6-C448-48F7-A766-001C65338E99 7010C337-829C-4F08-B6A4-1C8A9E943CBD
Our RAID 10 now only has three disks attached.

Rebuild

# diskutil list

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *119.9 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS ServerHD                118.9 GB   disk0s2
   3:                 Apple_Boot Recovery HD             784.2 MB   disk0s3
/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk1
   1:                        EFI                         209.7 MB   disk1s1
   2:                 Apple_RAID                         2.0 TB     disk1s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk1s3
/dev/disk2
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk2
   1:                        EFI                         209.7 MB   disk2s1
   2:                 Apple_RAID                         2.0 TB     disk2s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk2s3
/dev/disk3
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk3
   1:                        EFI                         209.7 MB   disk3s1
   2:                 Apple_RAID                         2.0 TB     disk3s2
   3:                 Apple_Boot Boot OS X               134.2 MB   disk3s3
/dev/disk4
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                                                   *2.0 TB     disk4
/dev/disk5
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Storage                *4.0 TB     disk5
thuja:~ pwladmin$

We can see in this list that disk4 has no partition maps and thus is the new disk. We can now add this disk into our degraded raid with:
sudo diskutil appleRAID add member <NewMemberDeviceName> <RAID UUID>
for example
sudo diskutil appleRAID add member disk4 7010C337-829C-4F08-B6A4-1C8A9E943CBD
Your disk is now part of the raid set. If you use diskutil appleRAID list you’ll be able to check the progress of the rebuild.

===============================================================================
Name: Thuja RAID A
Unique ID: 920F03EB-DE44-49AA-9934-0EF53EF032D1
Type: Mirror
Status: Online
Size: 2.0 TB (2000054910976 Bytes)
Rebuild: manual
Device Node: -
-------------------------------------------------------------------------------
# DevNode UUID Status Size
-------------------------------------------------------------------------------
0 disk1s2 D4BCB349-3255-473B-B586-EAF066C5BD6D Online 2000054910976
1 disk3s2 E01DB36B-CDC4-458C-AC07-507433DCB481 Online 2000054910976
===============================================================================
===============================================================================
Name: Thuja Stripe RAID
Unique ID: 9D9FEE5F-5F04-4051-A0AB-A985DFFAF2A0
Type: Stripe
Status: Online
Size: 4.0 TB (4000109559808 Bytes)
Rebuild: manual
Device Node: disk5
-------------------------------------------------------------------------------
# DevNode UUID Status Size
-------------------------------------------------------------------------------
0 -none- 920F03EB-DE44-49AA-9934-0EF53EF032D1 Online 2000054779904
1 -none- 7010C337-829C-4F08-B6A4-1C8A9E943CBD Online 2000054779904
===============================================================================
===============================================================================
Name: Thuja RAID B
Unique ID: 7010C337-829C-4F08-B6A4-1C8A9E943CBD
Type: Mirror
Status: Degraded
Size: 2.0 TB (2000054910976 Bytes)
Rebuild: manual
Device Node: -
-------------------------------------------------------------------------------
# DevNode UUID Status Size
-------------------------------------------------------------------------------
0 disk2s2 13AFF0CD-77FB-4E14-9A89-A09C01ACA4C4 Online 2000054910976
1 disk4s2 EAE79161-3729-41FB-81A1-97CE878C1E31 1% (Rebuilding)2000054910976
===============================================================================