sudo aptitude install mdadm raidutils
modprobe raid456

mdadm --create --verbose /dev/md0 --level=stripe --raid-devices=2 /dev/sda /dev/sdb
mdadm: chunk size defaults to 64K mdadm: array /dev/md0 started.

fdisk -l /dev/md0

Disk /dev/md0: **3000.6 GB**, 3000603639808 bytes
2 heads, 4 sectors/track, 732569248 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 65536 bytes / 131072 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System

parted /dev/md0

GNU Parted 2.2
Using /dev/md0
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) mklabel gpt
Warning: The existing disk label on /dev/md0 will be destroyed and all data on this disk will be lost. Do you want to continue?
Yes/No? yes
(parted) unit TB
(parted) mkpart primary 0.00TB 3.00TB
(parted) print
Model: Unknown (unknown)
Disk /dev/md0: 3.00TB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name     Flags
 1      0.00TB  3.00TB  3.00TB               primary

(parted) quit
Information: You may need to update /etc/fstab.

mkfs.ext4 /dev/md0p1