It's very confusing to users why they cannot edit track lengths on a release (which may be wrong) after adding a disc ID. They usually don't realise it's due to the disc ID.
I suggest that we add a tooltip (or better yet, some nice looking overlay) with wording something like.
"Track length cannot be changed as there are 1 or more disc IDs on this medium. Please set the track lengths from one of the disc IDs, or remove the disc ID(s) if incorrect."
If a fancy overlay; you could even link to the disc IDs page.