Constant writing of new Ubuntu HDD volume

[nry]

I am currently have the below in a test setup, but for some reason I can't quite get my head round is in nmon I see a consistent write to my RAID array of about 23-25MB/s on a new volume only just mounted.
There is nothing on this volume, I can't see any process which is using it (load is 0.01)

Hardware:
Asus P5Q-E
C2D E6750
8GB DDR2
Ati HD4350
120GB Boot drive
3ware 9690SA
- 8x Hitachi 5K3000 in RAID10

OS: Ubuntu Server 12.04.2 x64
Raid formatted to ext4

Does anyone have any idea what could cause this?
How can I investigate what's writing to the volume? (even though theres nothing on it!)

 

[patrickdk]

give it 10-24hours and it will stop.

 

[nry]

What is causing it? Never seen this before with any of my other arrays

 

[patrickdk]

Seems to be normal raid array init.

 

[nry]

That had crossed my mind but this is a hardware raid, which I wouldn't have thought would show up in nmon

 

[BecauseScience]

Run iotop and see what processes are writing at 23-25MB/s.