WDS Copying Windows Files in seconds!

Hi,

When performing imaging test (Windows PE 3 / WDS server 2008 R2), I find that sometimes the "Copying Windows Files" stage might last ~20 mins (I can see the multicast running). The next test I do, it runs only for a few seconds and goes onto "Expanding Windows Files" immediately.

In either case, the imaging process completes OK and Sysprep completes ok too.
I perform a willwipedisk as part of the unattend.
I am imaging down XP SP3.

So the quesiton is, WHY is it very quick sometimes??
I understand why it should be ~20 mins since it has to deliver ~10GB.
But how is it so quick subsequent times if I have wiped and partitioned the disk?


Thanks