I used to have CentOS 5.3 installed on the disk, and wanted to replace with Ubuntu, but I kept getting stuck at the “prepare partition” page.
It seems the drive is set up for RAID, even though I’m only using one disk. This seems to prevent the Ubuntu installer’s partitioning function from proceeding (or showing anything useful).
I followed the advice from here to open a shell from the live CD and type:
sudo dmraid -r -E /dev/sda
…and as I’m not using RAID (though JBOD’s the plan), I was happy to answer yes to removing the RAID flags. Installation moved on smoothly from then on.
Kudos to darkod.