Backup to new USB disk very slow (SBS 2011)

Hi, I'm on the trail of poor backup performance to a new WD 4TB USB disk with SBS2011 (Server 2008 R2). I've reformatted the WD disks using the WD utility to enable 512 emulation. Previous backups to 1TB USB disks were fine. It looks like I have hit the 512e problem. A 900M backup on the server itself is going to take weeks to finish!

As a test, thinking that a server 2012 might handle the 4K format better, I mounted one of the new WD disks (as D$) on a server 2012 Core, it's just a Hyper-V host, and started a backup from the SBS over the network to the disk on the 2012 Core. That backup is now 95% complete after running for 5.5 days (~2KB/Sec). For info, fsutil details from the 2012 system below

A backup of the hosted Server 2012 system, thats hosted by the 2012 Core, to the same USB disk shared from the 2012 HV Host ran fairly reasonably.

Any ideas?? KB982018 is already there.

Thanks

Ken

C:\Users\Administrator>fsutil fsinfo ntfsinfo d:
NTFS Volume Serial Number :       0x6012418212415de2
NTFS Version   :                  3.1
LFS Version    :                  1.1
Number Sectors :                  0x000000015d4affff
Total Clusters :                  0x000000002ba95fff
Free Clusters  :                  0x000000001edfd538
Total Reserved :                  0x0000000000000000
Bytes Per Sector  :               512
Bytes Per Physical Sector :       4096
Bytes Per Cluster :               4096
Bytes Per FileRecord Segment    : 1024
Clusters Per FileRecord Segment : 0
Mft Valid Data Length :           0x0000000000040000
Mft Start Lcn  :                  0x00000000000c0000
Mft2 Start Lcn :                  0x0000000000000002
Mft Zone Start :                  0x00000000000c0040
Mft Zone End   :                  0x00000000000cc840
Resource Manager Identifier :     EA4FA8CF-A57D-11E5-9CB9-3CD92BF19BDD