Linux RAID Mirror only half size

I just bought a couple 4TB WD Reds and for some reason when I try to mirror them the resulting array is only 2198.9 GB large.

Both disks are formatted as Linux RAID Autodetect (ID fd) with fdisk, and the command used to make the array is:

mdadm --create /dev/md/mirror0 --level=mirror --raid-devices=2 /dev/sdc1 /dev/sdd1

fdisk -l output:

Disk /dev/sdb: 4000.8 GB, 4000787030016 bytes
255 heads, 63 sectors/track, 486401 cylinders, total 7814037168 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: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1               1  4294967295  2147483647+  ee  GPT
Partition 1 does not start on physical sector boundary.

Disk /dev/sda: 120.0 GB, 120034123776 bytes
255 heads, 63 sectors/track, 14593 cylinders, total 234441648 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: 0x0002868b

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048   218292223   109145088   83  Linux
/dev/sda2       218294270   234440703     8073217    5  Extended
/dev/sda5       218294272   234440703     8073216   82  Linux swap / Solaris

Disk /dev/sdc: 4000.8 GB, 4000787030016 bytes
90 heads, 3 sectors/track, 28940878 cylinders, total 7814037168 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: 0xa36de11e

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1            2048  4294967294  2147482623+  fd  Linux raid autodetect

Disk /dev/sdd: 4000.8 GB, 4000787030016 bytes
90 heads, 3 sectors/track, 28940878 cylinders, total 7814037168 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: 0x8708ffe6

   Device Boot      Start         End      Blocks   Id  System
/dev/sdd1            2048  4294967294  2147482623+  fd  Linux raid autodetect

Disk /dev/md127: 2198.9 GB, 2198887792640 bytes
2 heads, 4 sectors/track, 536837840 cylinders, total 4294702720 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: 0x00000000

What am I doing wrong?

Your partitions are in fact only 2T in size - so the mirror created on top of them is similarly only 2T in size. The total sectors are almost double what are assigned to the partition.

Compare the information about the full device:

Disk /dev/sdd: 4000.8 GB, 4000787030016 bytes
90 heads, 3 sectors/track, 28940878 cylinders, total 7814037168 sectors

With that of the raid:

Disk /dev/md127: 2198.9 GB, 2198887792640 bytes
2 heads, 4 sectors/track, 536837840 cylinders, total 4294702720 sectors

Sectors are 512 bytes, so 512 * 7814037168 = 4000787030016 or 4T. Your partitions span sectors 1-4294967295 = 512 * 4294967295 or about 2.2T.
"Blocks" are 1k blocks, so you should see partition sizes in the neighborhood of 4000000000 blocks (more like 3.9 billion blocks actually) for your disk size.

The problem here is that you're using fdisk; it does not support partitions greater than 2T (or, more correctly, greater than 2^31 blocks). To create larger partitions, use parted instead.

If you rebuild the partitions using the whole disk and then create your mirror again with the same command you showed here and it should be fine.