Even though you own the original disk it's still illegal to download an ADF - you could have made an ADF of your own disk while it was still working, but downloading an ADF of someone else's isn't allowed.
Yeah right, so what is the difference between one disk image and another. If you own the disk, there is no reason you cannot use a working backup copy. Put it another way, how could you prove that the copy was not made from your own original?
