For the past month or so, TM has not been backing up at a constant or even a descent speed. Speed varies back and forth between fast-slow-stopped, but mostly very slow and stops on several occasions. Everything else feels right with the OS... Any ideas what's going on? Thanks. Specs: 2013 Ultimate MBA, OS 10.9.5 with an external 1TB USB3.0 HDD (C-S Journaled) with 50%+ available space.