I've installed a new nas/media center with Ubuntu 12.10with the following configuration :
- Mother board : ASUS F2A85-M LE (supposed to manage directly RAID1)
- Ubuntu 12.10 installed on an SSD
- 2x 3Tb that I would like to use for storage and home directories

So, in a first time I tried to use the RAID1 proposed by the mother board, I created it successfully, the bios seeing it correctly.
Once Ubuntu started, I don't see the RAID disk anymore, I see 2 distinct disk of 3Tb each.

I finally disabled the RAID from the mother board and switched to a software RAID.
To do that, I've :
- created a GPT partition table on each disk (using GParted)
- created an ext4 partition of 3Tb on each (using parted)
- build the array to create the RAID volume (using mdadm)
- finally mounted it in /home

This works perfectly except that when I mount the volume, it has a capacity of 2Tb.
root@ubuntu:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/sda1        48G  3.7G   42G   9% /
udev            3.6G   12K  3.6G   1% /dev
tmpfs           1.5G  924K  1.5G   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            3.6G  164K  3.6G   1% /run/shm
none            100M   52K  100M   1% /run/user
/dev/md126      2.0T  184G  1.7T  10% /home
There is the entry in /etc/fstab :
UUID=6508d199-5bdf-464f-b492-bfa69f3ea79f /home         ext4    defaults        0       2
Does anyone have any idea ?