A Microsoft file hosting and synchronization service.
Yes, I had the same experience when booting up a laptop that had been dormant for a couple of months...old file version uploaded and two months of work lost. Restoring earlier versions didn't work for the reasons everyone has repeated over and over. Now, every time I work on an Excel file in Office 365 I make my edits, save/synch to One Drive, and then save a copy of the newly edited file on an external drive. I pray that the One Drive developers never write code that searches and destroys every copy of a newly edited file across every connect drive (hmmm..., maybe I need to disconnect the external drive every time I save a copy of the Office file on it, just to be safe). I now am considering booting all of my old laptops far from my router and any stray internet connection, and deleting every Office file from the machines. Maybe having one (and only one) computer accessing One Drive will prevent my new files from being overwritten. Next, I'll have to figure out whether it makes sense subscribing to a service that promises one thing, and delivers another.