So I bought a second Seagate ST-225 "winchester" disk off ebay. I got it for the starting bid, nobody else made a bid! Result. I have connected it to my PDP-11 and used my "XXDP booting from ODT" trick to format it. Here are the results:
This is good news, the drive seems to be in good shape. Since then I have copied some RT-11 files to it and made it bootable. So I have a spare.