First, thanks to all of you for your answers!

Some software do exist (under DOS) to do the job of reading/writing/formatting
old 5"1/4 floppies in rx50 format ... with various level of success.

1 - rainbow driver

	raind112.zip
	or
	rx50.zip

    Those are drivers to make behave drive A: (NOT B:!) as an rx50. It is
    supposedly able to format 'em. With some luck it may work on a non
    rainbow PC. Didn't work for me.

2 - dos utilities

	22disk.zip
	or
	fdcdemo.zip

     They contains executables and drivers supposed to read/write at least
     rx50 format.
     I succeeded in READING an already formatted rx50 floppies but I couldn't
     write nor format any.


So, In cae you want to format/read/write rx50 floppies on your ol 5"1/4 drive
you have to tsarting point to investigate further!

good luck!

BTW my NetBSD-vax didn't end sadly without floppies: I've found a kind soul
    to format 'em on a VAX-VMS and put the boot/root images on. Now I'm
    stuck a bit further, but that's another story ...

-- 
Frederic R. ROUSSEL - frasc@cetia.fr