UNIX for Advanced & Expert Users

A large file is being downloaded in my harddisk. I want to copy it in my pen drive. Can i do that simultaneously without waiting for the download to complete?

Yes. But you are absolutely not guaranteed to have copied the entire file when the cp command ends for the disk -> pen drive.

Why - if the USB port moves data faster than the other process "downloading" the file can move data, then the USB can enounter a premature end of file by reading faster than the data ftp stream is being written and hitting the end of the file (from it's perspective) while the other process is still writing.

ya.. obviously the speed of flash drive will be faster. So is there any tool to achieve this? Or any tips to do this?
similarly I want to copy a file being downloaded to /tmp to my home folder(its mounter on another drive). This situation occurs often. Is there a way write some script or tool which will do the copy or move?

This might sound like a dumb question, but: why aren't you downloading to the target directory / the pen drive directly? Or are you downloading as part of a script? If so, we'd need to know how you download those files.

Say I'm viewing a flash video on a website and on the process, the content will be downloaded to /tmp. I want to save it from there than finding seperate pluggin or addon to download from the site itself.
Also when i'm downloading from dc++ or from browser , after initiating the download i find i dont have enough space in that drive, so have to move other stuff to other drive and put it back once download is complete. this is double the job. Thats why checking if there's some tool for this..