Limitations of RN51600

StephenB wrote:
But since the raw drive space is 20 TB, that means there are about 800 GB not accounted for. The OS and swap partitions account for some of it (about 30 GB), but I am not sure where the rest is.

The WDC WD4000FYYZ-01ULB0 disk is claimed to be pre-formatted with "Advanced Format" (See http://www.alternate.nl/upload/Advanced_Format.pdf). When I installed it fresh from the packaging I followed the installation instructions and if I remember correctly formatting by the RN516 is part of that. Could it be that RN516 doesn't support the "Advanced Format"? And does that explains the "loss" of 800 GB?

I have 6 disks, so I would expect to see 24 TB. Is this 6th disk standby, is it fully taken by the XRAID2 RAID5 or is there some other explanation why these 4 TB don't show?

(tebibyte, see http://en.wikipedia.org/wiki/Tebibyte)
(terabye, see http://en.wikipedia.org/wiki/Terabyte)

I think most hard disk manufactures list their disk sizes in TB (terabyte), rather than TiB (tetibyte). So my 4 TB disk drive has 4000000000000 Bytes (converted with http://www.unitjuggler.com/convert-memory-from-TB-to-B.html). And 4000000000000 Bytes = 3.6379788070917 TiB (converted with http://www.unitjuggler.com/convert-memory-from-B-to-TiB.html). Would that explain the difference?

Or does the difference in block-size in the df command explain the 800 GB difference?

df --block-size=1T lists 19
df --block-size=1TB lists 20

root@nas-FF-FF-FF:~# df --help
....
Display values are in units of the first available SIZE from --block-size,
and the DF_BLOCK_SIZE, BLOCK_SIZE and BLOCKSIZE environment variables.
Otherwise, units default to 1024 bytes (or 512 if POSIXLY_CORRECT is set).

SIZE may be (or may be an integer optionally followed by) one of following:
KB 1000, K 1024, MB 1000*1000, M 1024*1024, and so on for G, T, P, E, Z, Y.
.....

root@nas-FF-FF-FF:~# df --block-size=1TB
Filesystem     1TB-blocks  Used Available Use% Mounted on
rootfs                  1     1         1  38% /
tmpfs                   1     1         1   1% /dev
/dev/md0                1     1         1  38% /
tmpfs                   1     1         1   1% /dev/shm
tmpfs                   1     1         1   1% /run
tmpfs                   1     0         1   0% /sys/fs/cgroup
tmpfs                   1     0         1   0% /media
/dev/md127             20     2        18   7% /data
/dev/md127             20     2        18   7% /home
/dev/md127             20     2        18   7% /apps
root@nas-FF-FF-FF:~# df --block-size=1T
Filesystem     1T-blocks  Used Available Use% Mounted on
rootfs                 1     1         1  38% /
tmpfs                  1     1         1   1% /dev
/dev/md0               1     1         1  38% /
tmpfs                  1     1         1   1% /dev/shm
tmpfs                  1     1         1   1% /run
tmpfs                  1     0         1   0% /sys/fs/cgroup
tmpfs                  1     0         1   0% /media
/dev/md127            19     2        17   7% /data
/dev/md127            19     2        17   7% /home
/dev/md127            19     2        17   7% /apps

Update 2014-11-27 (from http://www.wdc.com/en/products/products.aspx?id=580#Tab3):

WD wrote:

As used for storage capacity, one megabyte (MB) = one million bytes, one gigabyte (GB) = one billion bytes, and one terabyte (TB) = one trillion bytes. Total accessible capacity varies depending on operating environment. As used for buffer or cache, one megabyte (MB) = 1,048,576 bytes. As used for transfer rate or interface, megabyte per second (MB/s) = one million bytes per second, megabit per second (Mb/s) = one million bits per second, and gigabit per second (Gb/s) = one billion bits per second.

I wonder if the bold italic text above explains the missing 800 GB?