I realize this is an old issue, but I have some insight on the problem I would like to share. My station (VP2/WL5.7/XP/USB) has been running and uploading nicely for 5 1/2 years now. I began syncing the WeatherLink folder to a flash drive in case the old desktop PC crashed. I have a netbook at the ready to take over as weather station server. The sync was set to continuous so that whenever a file in the WL folder changed, it was copied to the flash drive. Immediately, I started getting the "Unable..." message and uploading would stop. When I disabled the sync, the problem ceased.
I believe that the sync software (PureSync) "watches" files in the folders to sense when they are changed. Weatherlink may be interpreting this "watching" as the file in use by another program. The solution was to stop continuous syncing and switch to scheduled syncs. I picked times a bit distant from uploads. Using web-based backup services (Carbonite, Crashplan, etc.) might also cause the problem.
Perhaps someone else might find this useful.
