I've rebooted serveral times but nothing was changed.
Norman Gaywood:
There may be a process with a file open, using all the space, that has no name in the filesystem. For example, a process that might be out of control, writing to /tmp/logfile. The /tmp/logfile grows very large so you delete it. The process still has this file open and continues to write to it even though the file has no name. du would not show space used by this program, df would. To reclaim the space you have to kill the process.
If this is the problem, rebooting will also free the space. But I hate to recommend rebooting to fix problems.
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