This has happened to me twice over the past few months: both times I managed to identify the Apple Photos cache (I don’t have the name handy) that every day put another 62.8 megs of data into a library folder until it ate up the entire disk. Every day, always the same amount. Google said it was a photo-linking project: names and faces, something like that. about 75 GB of stuff by the time I cleared the folder out. I should have written down the daemon and the folder for reporting back: will do the next time it happens.