I have setup my Lightroom to do a periodic backup to my NAS.
Today it refused to do so, claiming that the target drive was full. Naturally, I assumed that this was some NAS issue, although there was plenty of room.
My next step was trying to backup to my C-drive. This resulted in the same error message.
Turns out that my c-drive (flash) had only 3-400MB of room to spare. Got rid of some junk on the c-drive, and now I could backup directly against my NAS.
Bottom line: I am okay with Lightroom using c-drive as a temporary storage for exporting, but the error message should have been clearly telling me that it was this drive (and not my chosen target drive) that was full.