there are many grey areas (can you copy one song? two? what if it’s your sister’s cd? what if she’s married? can you copy from someone, intending to erase it later? etc, etc, etc) As in all things, ASK your Rav, and follow what he says. That is your Da’as Torah. Good Luck.
P.S. I think this is such a big nisayon for our generation. it’s so easy to do, noone has to know, so many do it… but in most/many cases, it’s assur!