Hi @jamesbond and @Flash . Please accept my apologies for wasting your time. The problem is one of operator error somewhere. I also should have edited my previous post, rather than just the first one, to note my suspicions regarding this.
Some checks done by changing the /etc/hosts file on some Fatdog64-812 setups including one on an SSD that is using a savefile, another USB drive with straight multisession (save interval not set to zero) and the same USB drive with multisession with save interval set to zero (after having deleted the multisession files from the earlier test), all worked fine.
As I had started to suspect, I have managed to do something wrong somewhere with respect to the USB drive that presents the problem. I was complicating things, and am not entirely sure of what I had done. So I think I will start again from scratch.
Again, I apologise for posting too quickly, for not editing the previous message, and for the inconvenience.
Jamesbond, I hope you haven't gone far with checking things. I had hoped I would intercept any action until I had checked further.
For info only, the new host name in /etc/hostname was as I had changed it when setting up the save. I had been attempting to use a modified /etc/hosts file. I think my poor wording regarding some notes I had made in the sample hosts file (from Fatdog64-811, I think) might have caused confusion. I was referring to a suspicion that the first line of the /etc/hosts file might be automatically overwritten with the hostname from the /etc/hostname file. However, I cannot recall where I might have got that idea from, so I could easily have it wrong.
In an earlier "episode", that I thought I had cleared, I had copied the sample hosts file that I use to /etc, but with the wrong owner and permissions (even though I had a note to check them, D'oh). I thought I had cleared that, but maybe I hadn't gone far enough. I will try checking the session sfs files as you suggest, just out of curiosity, but I think a start from scratch, and then try to stay alert (ha, ha), and keep things tidy, is the only way that I am personally going to be confident that I have done things right.
Thanks and apologies again from the Fool In The Hollow.
Further info- I checked the multisession sfs files, as suggested, and the updated (from sample) /etc/hosts file appeared to be present. However, I noticed that the names of the later sfs files were missing "fd64save" in the names.
Which makes me think that I have been trying to be too clever and too greedy for my own good, which, through my stupidity, has unfortunately impacted others.
Previously I had placed the multisession saves in partitions. This time I thought I would try putting them in directories in the one partition (I set up different sets of multisession saves for different internet purposes). To add further complication, I copied the (uncombined) save files from one "set" as a base for another. In other words I think I have been doing my best to set myself up for a fall, particularly since I wasn't really alert when trying to do this. However, since everything had seemed to be working until I came up against the /etc/hosts file problem, I thought it was all right, and reacted the wrong way by not checking further before posting.
Hence, being dull and stupid, I posted too early, without checking whether the problem might be related to the "experimental" nature of what I was attempting (which really irks me, since I should know better, but keep doing this sort of thing).
It doesn't help, but I cannot apologise enough. Thanks.