Hi Stan,

  

(3407028224 sectors * 512 bytes per sector) / 524288 (chunk bytes) =

3327176 chunks

Right - more for clarity, these are the 512 byte sectors, not the 4k 
ones (otherwise I would have had a 12 TB drive :) )

Please show the exact iostat command line you are using and the output. 

iostat -x 1

quoted

Also, there is no other disk usage in the system. All the data is
currently on the NAS (except system "stuff" for a quite firewall)

I just spotted another thing, the two drives are on the same SATA
controller, from rescan-scsi-bus:

Scanning for device 3 0 0 0 ...
OLD: Host: scsi3 Channel: 00 Id: 00 Lun: 00
       Vendor: ATA      Model: WDC WD20EARX-008 Rev: 51.0
       Type:   Direct-Access                    ANSI SCSI revision: 05
Scanning for device 3 0 1 0 ...
OLD: Host: scsi3 Channel: 00 Id: 01 Lun: 00
       Vendor: ATA      Model: WDC WD20EARX-008 Rev: 51.0
       Type:   Direct-Access                    ANSI SCSI revision: 05

Would it be better to move these apart ? I remember IDE used to have
this issue, but I also recall SATA "fixed" that.

This isn't the problem.  Even if both drives were connected via a plain
old 33MHz 132MB/s PCI SATA card you'd still be capable of 120MB/s
throughput, 60MB/s per drive.

quoted

Thanks again,

You're welcome.  Eventually you get to the bottom of this.

And the email :) I now have the drive with no data on them, so I can 
even run write tests. I am going to start with the usual "dd" tests, any 
other that you would like to see ?

Cheers,

Pieter