Thanks for the quick replies!
Furrynutz,
I'd prefer not to upgrade, as this can cause problems. There's no mention in the changelogs (that I can find) that my disappearing folders problem - or indeed any remote access problems - are fixed in later versions of 320L system software.
Ivan, here's some more info:
1 Yes, I'm using the 320L's remote backup, with encryption turned on. The backup runs successfully (see my OP point 6)
2 Win 10. However, Windows is not involved in the transfer, so the version of Win should make no difference. It is an rsync/ssh transfer from one Linux machine to another (320L runs Linux). The local Win 10 PC is only looking at the mybackups folder; it plays no part in the transfer.
3 No, the folder does not reappear. Of course Windows takes about 5 - 10 minutes to refresh network shares, unless you reboot. Even after a reboot the folder does not appear. Also see my next point:
4 No, it does not appear in My Network Places. In any case, it has disappeared - as a network share - from the 320L, (my OP point 4) so it is unlikely to appear in Windows.
5 I used "mybackups" to make my OP easier to understand. The target folder is actually called "tech". However, Window's likes or dislikes for folder names should be irrelevant - see point 2 above.
My own gut feeling about the disappearing folder is that ownership and permissions on the NAS backup target folder (mybackups or tech or whatever) are changed by the transfer, such that all of the 320L's users have "access denied" rights to the folder. Somehow, restarting the 320L fixes this. (Perhaps the NAS detects "foreign" ownership or permissions and changes them to some default value.) Unfortunately there's no way to examine Linux folder or file ownership and permissions on the 320L. There's also no way to see what rsync switches the 320L is using when it runs the transfer.
In theory, ownership or permissions shouldn't be changed. The remote Linux PC has a user, "tech", with a password. The same username and password are set up on the 320L. This is the username and password used in the 320L's remote transfer settings. The remote Linux PC has private and public RSA keys set up for user tech. I presume these are used in the transfer, which is *successful* (see my OP point 6). However, although the 320L may be logging into the remote Linux PC as tech, it may be running the transfer (ie, the 320L's SSH/rsync programs) as root.
The fact that the backup folder can be seen in "My Files" is probably because I am "admin" when I look at "My Files". I am not any of the users I've set up on the 320L. On the other hand, I'm "admin" when I examine/modify a user in Management - Account Management, or examine Network Shares, but the backup folder has vanished (my OP point 4). All very mysterious.
Over to you...