I am (also) running lucene unit tests. In the teardown-method(@After) I (try to) delete the complete directory-folder. Unfortunately this does not always work. If not, the file _0_nrm.cfs (or _0.fdx) is the first to cause problems, i.e. is being "locked"... I do explicitly close the writers/readers and the directory itself. I even tried to unlock the directory.
What else could be "sitting" on the directory?