If it would help, I had to go through versioning history of all files, so have some more details as follows: Files were originally copied 10 days back. On BTSync syncing today, all new files (younger than 10 days) were copied OK, but the 4 files which were updated in meantime were wrongly synced. Since there were actually 4 error messages from the log quoted above (on the "newer" machine), 2 IO Errors and 2 ReadFile Errors, if they are actually 4 separate incidents, not 2, it may give hints on the underlying issue. And BTW, all 4 of those were XLS files. Also I am certain that for sure not all those 4 XLS were open at the time of the errors.