What's (where's) The Cache?

OK, here's a break from those difficult to solve problems. I've got a PC that's doing something I like. When I build a new system I'd like the feature in that one too.

When I close Outlook 2007 I back up the PST files including some PST archive files from C: to my data drive D:. I use GoodSync to control the transfer. I am backing up about 3GB. The transfer takes place far faster than the receiving Hitachi Ultrastar 2TB drive can cache/write. Sometimes I see a transfer rate of over 1 GB/s; hardly ever less than 350 MB/s. The Ultrastar remains very busy for 10-15 seconds after GoodSync says the transfer is complete. The source C: drive is a OCZ VTX3MI-25SAT3-120G. I have 16 GB of DDR3 1333 RAM. My system components are detailed in my profile.

In what possible way could the caching be implemented? I would love to be able to set it up in a new machine to work the same way!

Thanks - Dave