Comment 2 for bug 591721

I had this happen to me last month. My setup is similar, with two drives with RAID1. In addition, I'm using LVM on top of everything else, but I don't think that effects the root issue. My workaround was basically to use cfdisk to make the primary partitions. The exact steps were:

    * zero'd out both drives and removed both partition tables
    * ran the Ubuntu installer to create a normal primary 20 Gb partition on Drive A and installed the OS
    * booted into Linux, ran cfdisk on drive B (cfdisk -z /dev/sdb), wrote the partition table, and rebooted just for good measure
    * then ran cfdisk /dev/sdb again, and created two partitions (one 2 Gb boot partition, and the rest in another primary). I created an ext4 filesystem in the larger partition.
    * next, booted off the USB drive, mounted the new partition and tar-piped everything from the installed partition into the new
    * then zeroed out the partition table on drive A (/mnt/sbin/cfdisk -z /dev/sda), rebooted into the installer, mounted drive B, and ran cfdisk to create the partitions on drive A.
    * once the primary partitions were created, I used the installer to set up RAID1 and LVM.

After I manually created all my primary partitions, I was able to use the installer for everything else. It certainly seems that the installer's disk partitioner is broken in this case.