Windows 2012 Autounattend still asking about disk configuration
Solution 1:
On MDMarra's suggestion in Chat, I added a system partition to the manifest.. Something required for EFI compatibility, apparently.
This, now, seems to work.
So it looks like Windows 7 has no specific requirement to have a System Partition (although it will be created automatically in a non-unattended installation), whereas Windows Server 2012 will fail to install without a system partition.
Creating the partition is as simple as putting 0x27
in the TypeID box for the partition.
<DiskConfiguration>
<Disk wcm:action="add">
<CreatePartitions>
<CreatePartition wcm:action="add">
<Order>2</Order>
<Type>Primary</Type>
<Size>71800</Size>
</CreatePartition>
<CreatePartition wcm:action="add">
<Order>1</Order>
<Size>512</Size>
<Type>Primary</Type>
</CreatePartition>
</CreatePartitions>
<ModifyPartitions>
<ModifyPartition wcm:action="add">
<Active>true</Active>
<Format>NTFS</Format>
<Order>2</Order>
<PartitionID>2</PartitionID>
</ModifyPartition>
<ModifyPartition wcm:action="add">
<TypeID>0x27</TypeID>
<PartitionID>1</PartitionID>
<Order>1</Order>
<Active>false</Active>
<Extend>false</Extend>
<Format>NTFS</Format>
</ModifyPartition>
</ModifyPartitions>
<DiskID>0</DiskID>
<WillWipeDisk>true</WillWipeDisk>
</Disk>
<WillShowUI>OnError</WillShowUI>
</DiskConfiguration>