After a quick glance thru the TUHS.org archives, I found a quick & dirty 
hack for 4.0-Stable by Jason T. Miller <[EMAIL PROTECTED]>
The README for this thing is as follows:

 >This tarball contains a dumb hack to read and write DEC RX50 diskettes
 >under FreeBSD. It consists of two pieces, a kernel patch and a set of
 >filters. The kernel patch, which should be applied to SYS/isa/fd.c, >adds
 >the RX50 physical format to the FreeBSD floppy driver. The patch is >based
 >on FreeBSD 4.0-STABLE, your mileage may vary. However, it is >conceptually
 >simple and should be easy enough to apply by hand. Note that this >format
 >is identical to the 5.25" 800K format, but with only one side.

 >Also in the kernel/ directory is a patch for MAKEDEV which adds device
 >nodes for the new format, with the name fd[n].rx50. Note that using >this
 >node with a drive that is not a high-density 5.25" floppy results in a
 >"device not configured" error.
 >The filters/ directory contains two filters, rx50in and rx50out, which
 >deal with the logical sector interleave performed by the RQDX >controllers
 >on the PDP-11 and VAX; ideally, this would be handled in the driver; >like
 >I said, this is a dumb hack. Note that the filters read or write the
 >_entire_ disk; short input results in null-padding. This shouldn't be a
 >big deal, but it does result in a bit of extra disk I/O. C'est la vie.
 >They use standard input and standard output, and no output (except for >an
 >error message on standard error) is created if the input exceeds the
 >capacity of an RX50. Also keep in mind that non-PDP, non-VAX
 >implementations of the RX50 used different layouts, so the filters are 
 >appropriate, for example, for DECmate or DEC Rainbow disks. The kernel
 >patch is, however, and this is the sole advantage of doing the >interleave
 >in userland.
 >Create a tar archive of 'directory' on an RX50:
 >  tar cf- directory/ | rx50out >/dev/fd1.rx50
 >Extract a tar archive from an RX50:
 >  rx50in </dev/fd1.rx50 | tar xf-
 >Finally, you can try to format a disk as an RX50 with
 >  fdformat /dev/fd1.rx50
 >I say 'try to format' because the diskettes I format in this fashion >are
 >prone to read/write errors on a real RX50. This is probably due to head
 >alignment issues, but it could, of course, be something else. I've had >no
 >problems reading and writing disks formatted in other ways (by DEC or
 >using custom hardware, in my case a Shaffstall 6000 media conversion 
 >Bear in mind that RX50s, though they are written at 96tpi and thus 
 >a high-density drive to read and write on a PC, are _not_ a >high-density
 >format, and using HD media will likely result in _lots_ of errors. I >use
 >96tpi double-density media myself, but good luck finding it. The best
 >substitute that is commonly available seems to be standard 360K
 >double-density media without hub rings. Once again, your mileage may 

 >Please send any questions or comments, bugfixes or patches to
 >[EMAIL PROTECTED]; once again, this is a dumb hack, written in an
 >evening, full of sound and fury, signifying nothing. The code is ugly, 
 >the results, I'm happy to report, are not.
 >-Jason T. Miller
 > June 9, 2000

Hope this helps , Martin Heller

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