Article 159943 of comp.os.vms:
In article <573a14$fsf@nicaragua.earthlink.net> Mark Honeycutt,
mfhoneycutt@earthlink.net writes:
>Subject: RD53 Jumper Settings
>I noticed that on the back (connector) side of the drives in the 
>center about 1/2" inside the drive are six jumpers that come up
>tell me what these jumpers are for and what their possible settings
>are?

I recently had some experience with these jumpers.  The six jumpers from
left to right represent ds4, ds3, ds2, ds1 and two others that I don't
recall.  In my drives, the far two right jumpers are installed.  ds3 is
installed  to select drive 0 and ds4 is installed to select drive 1 (at
least according to some old usenet discussion).  I found that installing
a jumper in either ds3 or ds4 made no difference when booting the disk in
a PDP-11/23 box.  In each case, the drive came up reporting itself to be
du0.  When I moved it to another box with a PDP-11/73 and existing RD54,
I could get it to work in only one configuration - setting the RD53 to
drive 1 (ds4) and the RD54 to drive 0.  The reverse combination did not
work.

Good luck - most RD53s only last a few years in between the need for
reformatting, unless they catastrophically fail first.