Q: Installing Build 71 instantly deleted about a third of my SkyDrive files in no detectible pattern. Accessing the drive through live.com confirms the disappearance. It's too bad, because despite bugs and occasional erratic behavior I had found the previous build to be invaluable.

A:
After investigation, it was that the files were still sitting in build 60's cache (some build 60 bugs that either failed to upload or for some other reason).

Build 71 has a different cache format so it feels like the files disappeared. What happend was that the files were not uploaded to SkyDrive in build 60.