The process has the same PID, unlikely it gets restarted. I can reproduce this behaviour on all computers we're using! sync.log is reset after about 10MB / 15minutes of data. I've had a closer look with process monitor, seems like there are two calls which might cause this issue. One is definitely coming from btsync where sync.log.old is created, but there's another one from svchost.exe. Not sure why the service process does that though.. UPDATE: Couldn't upload my export of process monitor, you can find it here http://www.mesch.ch/sync.log