There may be some corrupt projects in the library that is causing the error message. I believe that Error 36 is an input/output error. It can be caused by a corrupt file or possibly and incorrectly formatted external drive.
So you can continue working, try creating a new library on the external drive and then copying (not moving) this one project over to it. You can copy the project by moving your cursor over the project's title in the Projects browser view, and then selecting Copy to Library from the drop down menu that appears. Both the original library and the destination library must be open for the Copy option to be displayed. Before copying the project, click on its icon in the Project browser screen and do a File/Consolidate Project media. The external drive should be formatted Mac OS Extended (Journaled). If you need to reformat be aware that reformatting erases all data on the drive, so first back up anything that you want to keep.
You can do the above process with the rest of your projects until you find the one that possibly is corrupt.
-- Rich