Putting in the new drives was pretty self-explanatory: open the bay, plug them in, open the system setup (press F2 upon system boot), and make your way with virtual drives. I configured the two 4TB drives in RAID0 as a single drive. This was just the beginning...
- initialized the 8TB drive with DOS partition table,
- ran cfdisk, made a single LVM2 partition with "100%" capacity (I was just hitting 'Enter' repeatedly :D ) and didn't bother to double check its actual size after partitioning,
- pvcreate,
- vgextend,
- lvextend (to the 2TB 'data' partition),
- found out that the resulting partition is only 4TB,
- found out that additional LVM physical volume was only 2TB big,
- vgreduce: failed,
- ...
- ...
- more screwing up
- ...
- ...
- yanked out the new drives,
- reboot: broken volume group,
- removed the broken logical volume from broken volume group,
- realized that only GPT can handle partition larger than 2TB
- redoing everything with GPT: disklabel with parted, pvcreate, vgextend, mkfs,
- ...
- working 10TB partition