Ok.... Pity you didn't choke.....
Each time you add to your hard drive it reformats itself and the data stored. It basically reorganises itself to be has efficient as it can.
Any out of place data, surplus to requirements or left over during the reorg gets discarded this data by the way is your music.
It is Incey wincey bits we are talking about and can go unnoticed but one day you'll put the original CD on and it'll sound better.
The data on the CD is fixed. it'll never be reformatted, squished to fit on your hard drive or be subject to other issues found with ref to data storage.......
Now like I said before I have nothing in way of evidence to prove this - it's how these devices/computers work.
It's a bit like the billions of pounds floating around in cyber space made up of tiny fractions of a penny...... Lol..... Sorry for laughing but , just watch superman3.
The music gets lost.