There are multiple related issues here.

 - partman-md does not support assembling a RAID1 array out of a set of whole 
disks; it only supports assembling an array out of partitions.  It *should* 
support using whole disks, so that you have the option of partitioning the RAID 
array and having those partitions show through on the underlying block devices 
in a way that is meaningful to the firmware.
 - in the absence of such support in partman-md, users are led down the garden 
path *thinking* they're creating arrays out of the disks when they're actually 
creating full-disk partitions on the component disks and RAIDing those.  
partman-prep should warn the user *early* (i.e., at the partitioning stage) 
that they've created themselves an impossible-to-boot partition layout, and 
guide them to go back to fix it.
 - grub-installer should support installing to multiple PReP partitions in 
parallel for proper full redundancy in the face of a disk failure.  This can be 
achieved in one of two ways:
  - the user can create a RAID1 of two partitions on different disks (e.g. 
/dev/sda1+/dev/sdb1 -> /dev/md0), and grub-installer can support installing to 
this as a PReP partition; or
  - grub-installer can detect multiple PReP partitions (possibly requiring the 
user to manually specify each one that they want to use) and install the 
necessary boot record to each of them in turn.

The 2nd and 3rd issues should be relatively straightforward to fix.  The
first issue is going to require some redesign of partman-md and is not
something we'll want to take on for 15.10.

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https://bugs.launchpad.net/bugs/1487365

Title:
  Ubuntu14.04 .3 RAID installation fails on firestone

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