disk space useage show 90.1 mb out off 500mb why ?

The uploaded file could not be moved to /home/prostyle/public_html/wp-content/uploads/2011/11.

Warning: Unknown: write failed: No space left on device (28) in Unknown on line 0

Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0