How can you temporarily disable file caching in Windows XP?

Archived from groups: microsoft.public.windowsxp.customize (More info?)

I am in need of a way to temporarily disable the Windows file caching that
runs whenever you copy files from a media source to the system. By default
Windows caches this data, and on subsequent copies of the same source, it
pulls from the Windows cached files, rather than from the orginal source
media. I need to disable this so that Windows goes back each and everytime
to the files on the original source media. We are testing disk read/writes
with a product and are using an SD card as the removable media. We have
tried using different sized files including very large files, but as soon as
Windows XP copies all the files through once, it caches from then on. We are
using the copy command from a command window, and realize will probably have
to convert to .NET scripting or WSH. Can anyone advise on how we can do
this?? Ideally we were looking for a switch or registry key that we could
enable or disable that would allow us to temporarily stop file caching from
"removable media" since disabling it for system operations would be very bad!
I appreciate any pointers anyone has to resolve this problem.