What would cause this file transfer slowdown?

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http://serverfault.com – This screenshot is of me transferring a large file (a .VHD) from one logical volume (made of 2 spindles in RAID0) to another one (made of 2 spindles in RAID0) on the same physical machine. (This is not production so don't worry about the sanity of that hardware configuration.) I initiated the transfer via explorer.exe on a third machine on the same LAN, so some TCP protocol may be implicated in the crime here; I don't know. As the file transfer starts, it peaks out at around 220MB/sec, which is the speed I would expect from a volume to volume transfer on this machine, and then right about 50% (HowTos)