I wanted to install to a portable hard drive simply so that I could have all my documents, music, videos, etc all on the same drive as the Puppy, instead of fiddling around with 2 or 3 drives. But it's not having it.
I do the first couple of steps and then an error message comes up telling me there's nothing plugged in. I've tried with 3 different Puppies and with 3 different drives, same thing happens every time. I know there's nothing wrong with the drives, 2 of them have had other distros installed previously, one of them still has another distro on it, and no problems have arisen. It'll recognise any USB stick I plug in, but not an external HDD.
I'm sure this issue has come up before from somebody else, but I can't find it now and it was quite a while ago, so may even have been on the old forum.
I've found a work around, I've put the ISO on the drive using YUMI, which for the benefit of anyone who's interested I found more convenient than Ventoy. It was slightly more fiddly initially, but once it was done there was no problem creating save files, the save was recognised straight away when I rebooted, and the rest of the drive was free for me to do what I wanted with. It works for me.
I would still like to know what the issue is though, with installing straight to an external drive.