The folders imported from Outlook would have been in Local Folders. Help/Troubleshooting, Profile Folder, Open Folder, close TB, open Mail/Local Folders, delete the .msf files there and in any sbd subfolders. Restart TB - do you see the missing folders under Local Folders in the Folder Pane?

I fear not... Al I see under Local Folders is Trash and Outbox, both empty. No sign of My Documents, although I do have a folder on the server named Holding (which I set up myself, it's not one of the standard set of folders) and that is there, all complete. Perhaps that could form a backdoor for My Documents etc?

Did you look in Mail/Local Folders with File Explorer, as explained above, with TB closed? Looking at Local Folders in TB is not going to show missing folders.

Yes, I followed your instructions to the letter (I think!)

Starting with TB 31.8.0: Help>Troubleshooting...Open Folder, close TB.

Using File Explorer in the FE window opened by the Open Folder above: delete all the .msf files (as you said).

Install TB 78.5. Open that installation. Neither Mailstore Export nor My Documents visible. Close TB.

Reinstall TB 31.8.0 and open it. Mailstore Export and My Documents reappear.

What am I not doing/doing wrong?

Help/Troubleshooting, about:profiles. How many profiles are listed, and what are their names? I think the folders you saw with TB 31 are in a different profile from the one that's used by TB 78.

See attachment: I can't seem to get to the Profile folders from 78.5.0.

The default-release-1 profile is the current default. Click Open Folder for the Root Directory of k3jlivoa.default, and open the Mail/Local Folders subfolder, where I think you'll find the imported Outlook folders. Copy the mbox files there - the large files with no extension, named after folders - to Mail/Local Folders in the default-release-1 profile, while TB is closed. Restart TB and see if the folders appear under Local Folders in the Folder Pane.

Did exactly and only all that, but no banana.

So:-

Copied the .sbd folders, the un-suffixed files with the same names and all the .msf files, and everything appeared where it should be!

So far so good, many thanks. Now what about the Address Book and the calendar from 31.8? I can't identify anything in k3jilivoa.default that looks like a candidate for copying?

The address books in the k3... profile are abook.mab (Personal Address Book) and history.mab (Collected Addresses). When you copy them to the new profile, then restart TB, they will be backed up and converted to sqlite files. The calendar-data subfolder of the profile contains the calendars.

Thank you for all your help, it is much appreciated. However, I have bitten the bullet and reverted to Microsoft 365, even though it costs more in cash terms: 1) it feels like coming home 2) it works straight out of the "box" - I had no trouble at all copying My Documents from TB to my Holding folder on the email server then copying it back in Outlook, and it took all of three hours to do the lot 3) the rest of the Microsoft 365 canon also works like silk.

The significant difference between moving from Outlook (even the old version) to TB and moving back to (new) Outlook is that getting TB to work took all the help that you gave me over several weeks whereas the return journey needed no help at all and took half an afternoon. Why did the TB upgrade process not copy all the files you identified for me, automatically and without all the fuss? I suppose it comes down in the end to 'you get what you pay for' which in the case of Microsoft is 'quite a lot'.

I think your issue all boils down to your repeated use of the term "my documents" like as if it is something tangible. Is was a folder in Windows XP. So I actually had real difficulties understanding where this was going at all.