Windows Xp and file copy

This morning I select some folders from my external HD, CTRL+C and then CTRL-V in a folder of my secondary disk. The external disk begins to read at full speed, the OS shows me a windows that tells

Preparing to copy operation

That windows stands on my desktop for 20 minutes before it begins actually to copy the files….. The real copy operation take 20 minutes. So I’m asking myself, “why the OS spent 20 minutes preparing the copy?” This is the reason why quite often I use XCopy from a dos prompt to copy large amount of files.

Alk.