Bert Coules said:
Berton, thanks for that. I shall definitely look into mapping, which is an area I've never used. But as far as my immediate problem goes...

Sean, I solved the accessibility problem by tweaking the permissions in the Security settings of the PC and the specific drive and folders I needed to get to across the network. The result is that I now have the setup I wanted, with each machine allowing access to the other. Many thanks again for all your help and patience.

I now have something else to look forward to: I'm thinking of replacing the main desktop PC with something a bit newer and faster and with more storage, and this is going to involve moving more or less the entire contents of the old computer - not only data but also programs, settings, everything - to the new one. I'm hoping that it's going to be a nice, straightforward procedure...
I've done the same, got a new desktop computer since none of mine would properly meet the System Requirements of Win11. It's a Dell Vostro tower, came with Win11 Pro and a 512GB NVMe SSD drive but has a bay for a 3.5" so I plugged in a 2TB HDD, system works good. I tend to use the Core i5 CPUs but do have a Win10 Pro with an AMD Phenom II on a Gigabyte motherboard in a desktop. Also have a custom that was given me for parts with a Gigabyte motherboard with an Intel Core i7 CPU but got it fixed up and it's running Linux Mint. Also have a good Dell Precision with a XEON CPU and Win10 Pro that was given me. I won't let computers be put in the landfill until I salvage parts I might need.