If anyone is interested, I have written and published a Python script that will update local copies of the M4 20 Hz/10 min data based on a) whether the local version exists and b) whether the online version has a newer modification date. The script works on Windows and should work on Linux with a few file-path modifications.
You can find the script (“update_nrel.py”) and a module with helper functions at http://people.duke.edu/~jmr64/update_nrel_data/.
This is an extended version of the code from the old forum post (http://forums.nrel.gov/t/how-are-you-accessing-updating-your-wind-data/724/1), in addition to being newly implemented in Python.
Please let me know if you have any questions or see any errors.