My motherboard has an Intel RAID controller. I set up 2x3TB HDD in RAID1 named "Home". This should make a single device that is about 3TB. This is what I end up with:
# fdisk -l
Disk /dev/sda: 240.1 GB, 240057409536 bytes
255 heads, 63 sectors/track, 29185 cylinders, total 468862128 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003431e
Device Boot Start End Blocks Id System
/dev/sda1 2048 3905535 1951744 83 Linux
/dev/sda2 3907582 468860927 232476673 5 Extended
/dev/sda5 3907584 468860927 232476672 8e Linux LVM
Disk /dev/sdb: 3000.6 GB, 3000592982016 bytes
255 heads, 63 sectors/track, 364801 cylinders, total 5860533168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00097a6c
Device Boot Start End Blocks Id System
Disk /dev/sdc: 3000.6 GB, 3000592982016 bytes
255 heads, 63 sectors/track, 364801 cylinders, total 5860533168 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00097a6c
Disk /dev/mapper/isw_djdcaefbcg_Home: 651.5 GB, 651543318528 bytes
255 heads, 63 sectors/track, 79212 cylinders, total 1272545544 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00097a6c
Device Boot Start End Blocks Id System
As you can see, I end up with a device that is 650GB, not 3TB. I'm using a fresh and updated install of Ubuntu 12.04.