If you find that you are seeing tracks that are highlighted red within your Engine Library this means the file path is either lost or corrupt.
To fix this you will either need to relocate the tracks or delete these from the collection.
If you are wanting to relocate these files you will need to do the following below:
(If on Windows or Mac you will need to select the destination this files exist which could be a custom location depending on where you saved the file too.)
See example below for a Windows PC:
If you want to clear these up manually you can do this by using the following methods below:
1) Hold Shift and select a group of tracks you want to delete within the Engine Library collection and the right click and select Remove From Collection.
2) Hold CTRL and select individual tracks you would like to remove from the collection that are mixed with tracks that are listed to be within your select. Then right click and select Remove From Collection.