Raid5 with 3 / 2TB drives shows only 2TB capacity buy fdisk reports 4

Debian Squeeze 64 Dell R510.

Can someone explain why df -h shows my raid 5 sata with 3 / 2TB drives shows up as 2.0T capacity but when I do fdisk -l it shows something different

me@scf:/mnt/raid5$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1             225G  4.8G  209G   3% /
tmpfs                 7.9G     0  7.9G   0% /lib/init/rw
udev                  7.9G  148K  7.9G   1% /dev
tmpfs                 7.9G     0  7.9G   0% /dev/shm
/dev/sdb1             2.0T  1.5T  479G  76% /mnt/raid5

and fdisk -f

Disk /dev/sdb: 3999.7 GB, 3999688294400 bytes
255 heads, 63 sectors/track, 486267 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x4d9f5ba6

Device Boot      Start         End      Blocks   Id  System
/dev/sdb1 1      267349  2147480811   83  Linux

Raid Controller

02:00.0 RAID bus controller: LSI Logic / Symbios Logic LSI MegaSAS 9260 (rev 05)

Is this something I need to worry about? Not sure what to do about it.

You've probably stumbled into a 32-bit/64-bit problem. Disks with the MBR partitioning layout are limited to a 2TB maximum drive size due to the field widths in the partition table and the block-size; the field is 32-bits wide, but with a 512 block-size 2TB is as big as it allows. You can get higher with 4K-block devices, but those don't appear to be used in your system.

GPT partitioning does allow larger disk sizes, but you need to use parted not fdisk to create that kind of layout.