====== Building an additional raid ======
My box already had two hard disks in raid10.  Wanted to add two more disks in a second raid.  Here is what I did.  This was on Ubuntu Trusty.

  * Boot with Ubuntu install CD
  * Go into recover mode
  * Navigate until you hit the partition disk option (had to go forward and backward in the wizard to end up here!)
  * Perform the partition as you would on a new system, except only do it for the two new disks and leave the existing untouched
  * When asked to specify mount point specify / (root)
    * Specifying root is not what I //actually// wanted as this was a supplementary data disk (set)
    * The install (even recover mode) would not let me proceed if I did not have a root partition mount
  * Confirm the write to partition table
  * The system will start installing the OS on the new root partition! Not what we wanted of course.
  * Press reset (Ctl-Alt-Del) in the middle of the install to force reboot!
  * Boot back into the system (as before) with your regular boot disk
  * Run ''fdisk -l'' to check that disks are raid are all in place - should be or something went awry
  * Use ''blkid'' to determine the UUID of the new raid disk set
  * Update ''/etc/fstab'' with mount point (create the mount-point directory first)
  * Issue a ''mount -a'' to mount the disks
  * Remove the frivolous install files that the Ubuntu installer started to copy in the new set of disks
  * Issue a ''cat /proc/mdstat'' to observe raid sync process
  * Issue a ''mdadm --detail --scan'' to get the new (and existing ARRAY) details
  * Update ''/etc/mdadm/mdadm.conf'' with the new ARRAY details
  * Run ''update-initramfs -u'' to add it to the initramfs image

I used the install CD as the process is standard and you can do it via a //tool// as opposed to commands, that I could not find a good comprehensive set of.  It is sort of imperfect in that you have to fool the installer into just creating your raid/partitions and force abort the actual install. However it works and does not have any risk.