Most probably another application (e.g. disk indexing, antivirus scan, backup, etc) opens the file and denies your application write access for a short time.
You can ignore this error retry since it for a short time. Another solution is to set the deny mode to "deny read/write" when you open the file. That is a setting on the "Open File" node. By default, LabVIEW opens file allowing simultaneous read/write access by other application. This is seldom required and wouldn't be a problem if the other application didn't lock the file for itselt the time it access it. Unfortunately, the high level File I/O VIs don't allow to set the deny mode on the file; you have to open the file using the "Open File" node directly.
