Copying a sequence or clips from one timeline to paste in another timeline

Thank you for your quick response, Tom. In each case, the media is stored inside the libraries. The reason for this being that I will eventually have to send the final edit to have the colour correction and audio mix done elsewhere. Therefore, in my case, this means sending an entire SSD enclosure away complete with all the original data required. Each library is located on its own disk. The initial library has assembled footage from a particular interview and the second library contains an off-line edit comprising of several sequences from various interviews copied from various libraries. As mentioned before, the off-line disk is getting loaded with numerous and useless clips and - even though I am working with a 4Tb SSD - I am worried about running out of space.

Well ...allright. If this is the way it has to be, then so be it. Still, it is strange that a relatively sophisticated application like FCP will not let you delete unused media files while moving an edit along. I know I can remove some render files and even some optimized media files, but it still leaves me with heavy leftover files on the disk. Why doesn't the system only copy the files pertinent to the sequence(s) you want? By the way, I'm working with simple HD files... Thank you again, Tom.

Thank you, terryb. Cannot work with proxies this time around. See my reply below to Tom's questions.