Problem moving module files to other directories

[Lorenzo Pesce]

Dear all,
I am building a relatively complex project where different
libraries share some functions. These libraries, and the
functions are in modules, which I would like to leave in
different directories (I work right now on an Appled Power PC,
and use gcc 4.3.0 20070810 (experimental)).
All was fine until I started to strip some modules from one
directory because they need to be used also by other libraries.
Then 
####
gcc -O2 -J../SRC_SYS -I../SRC_SYS  -c input.0.0.f90
input.0.0.f90:16.64:

use io, only: line_length, line_format ! size of the input line
                                                               1
Fatal Error: File 'io.mod' opened at (1) is not a GFORTRAN
module file
###

The same gcc built the module. The module calls another
module, but the module is also in the path with -J (taking out
 the -I doesn't help).

If io.f90 is in the same directory, no problem.

I have seen some threads about this, but nobody actually
answer the question of why it is happening and why can't it be
done or what to do.

The source compiles under any fortran 90 compiler (I tried all
of them, the exception is Lahey which is not ANSI so the
problem is on them) and it is perfectly good  code that worked
for a long time and still does.

Thanks a lot,

Lorenzo