Here’s one source from the FAQ section:
http://discourse.weather-watch.com/t/27619
There also has been lots of discussions on this subject…it requires correcting the log file and the data files using built-in functions of WD. Then you must re-create the graph file to remove the rain from showing on there. I know I have done this in the past and it does work if you do the sequence in order.
WD has a built-in backup routine and I recommend it, and for greater assurance you can store a copy on your server in case of harddrive failure on your WD machine.