Last August I switched from cloud back-up to a 4TB drive on my home network. The drive's been running fine, and incrementals have run consistently. I've occasionally used the Time Machine UI to retrieve old files - very slick. (Between our 2 computers we have ~2.5TB of capacity, btw.) The other day I noticed a message saying that Time Machine needs to create a fresh back-up or something. Whatever. I didn't pay it any mind and went about my business. Just now, I got a message that the back-up was now complete. Hmmmmm. I opened TM only to find that everything prior to Thursday night is gone. That history was wiped. I didn't think Time Machine worked that way. Note that nothing has been done to my computer or the drive or anything else on our network. Any ideas what happened?