Okay, after typing the above I realised I had a way forward. I backed up and then copied my render presets folder to my external drive. I then created an alias/shortcut to it and put that back where the original render presets folder was. Now Poser looks at the alias goes to the external non-SSD drive for the presets and gets them in alpha-magically-betical order. Hurrah! (yes, I did have to put a number in front of them to get them numerically ordered).