Logback can't roll log files to a different filesystem than the active log using TimeBasedRollingPolicy (and RenameUtil.rename()) on AIX and possibly other Unix/Linux-based operating systems. Per the Java 6 File.renameTo() method JavaDoc, "The rename operation might not be able to move a file from one filesystem to another". The JavaDoc for RenameUtil.rename() states that it should fall back to renaming by copying, but the code doesn't do that. The renameByCopying() method is never called.