Use visual mode, or directly:

    :10,20yank

Copy to a new file:

    :new | put | 0d

Usually, you'll either have a criterion, e.g. move all lines containing `pattern` to the end:

    :g/pattern/m$

To copy (:copy or :t)

    :g/pattern/t$

To yank to a register:

    :let @a="" | g/pattern/y A