(setf *read-default-float-format* 'single-float)

(defvar *num* (float (/ 1 3)))

(format t "~a~%" *num*)

(with-open-file (out "my.db" :direction :output :if-exists :supersede)
  (print *num* out))

(setf *read-default-float-format* 'double-float)

(with-open-file (in "my.db")
  (format t "~a~%" (read in)))