(TL;DR at the bottom) Today I tried to revert from the Public Beta back to Yosemite on my 2015 15" rMBP, the one in my signature. I did so by using Time Machine as I use that for backups. However I made a critical error, I did not stop El Capitan from backing up to that same Time Machine backup that I had previous backups of Yosemite on. When I attempted to restore to Yosemite today using those backups it did not work correctly. It shows me many different backups going back all the normal days as you'd expect and it lists the OS X versions on some of them as El Capitan (the newest backups) and Yosemite (the older ones). I naturally selected the Yosemite entries but because El Capitan had already backed up to the same Time Machine volume. it had altered all of the file indexes, making it incompatible with Yosemite. When I restored and logged in, about 50% of my files were not present. My iTunes library, documents, many folders, application settings etc were all gone. I restored it two more times, once from recovery mode (CMD+R on bootup) and once from Migration Assistant and both resulted in the same problem. I also tried to open Time Machine in yosemite using the normal Time Machine interface to just recover what I could manually but Yosemite is completely unable to read any of the backup entries in Time Machine, the format is just not compatible. So finally I rebooted to recovery mode and restored back to El Capitan and all my files were present back as they were originally. The moral of the story, don't be stupid like me and allow your beta of El Capitan to mess with your Yosemite backups, start a new backup instead. TL;DR: El Capitan changed my Time Machine format making reversion to Yosemite impossible without file loss. Start a new backup for El Capitan while testing it instead of using your previous Time Machine backup volume.