I've been using Time Machine without incident on the same external drive for years. Recently, I've noticed a problem where I sync with Time Machine, and the backup will finish instantaneously so that I know it didn't have enough time to actually do the backup. It will go from "preparing backup" to "finishing" immediately, with no intermediate stage telling you how many gigabytes it's backing up and what the progress is. I think I've only noticed this happening after installing the most recent beta of 10.11. Anyone know what the problem could be?