Data Transfers · Problems and messages
What does “A piece of the file couldn't be downloaded” mean?
Short answer: One part of a large download failed, usually from a dropped connection. Everything that arrived is kept; check your connection and start it again to resume with the pieces already downloaded.
One part of a large download failed — usually because the connection dropped or the server stopped answering for a moment. What arrived so far is kept, so the transfer can pick up where it left off rather than starting again.
If a piece arrived damaged or incomplete, it's simply fetched again; nothing already downloaded is lost.
Check your connection, then start the same download again; it resumes with the pieces it already has.
Related: Large downloads that resume