Did you save as a set of eml files or a single mbox file? Create a subfolder of Local Folders, then right-click it, IET NG/Import all messages from a directory, for eml files, or right-click Local Folders, IET NG/Import mbox file.

Well... I don't really know. Whatever type file IET NG creates when you mash the 'save all mail files' button. It's a single file folder but doesn't have a file attachment in the name, other than .default.

It looks like you saved the entire profile, if the result was a folder with a name ending in .default. Check the IET NG Options to see the choice for Backup content on the Backup scheduling tab. If it's 'All profile files', then Tools/IET NG/Import profile should import the folder.

None of this is working. I don't really understand the 'save all mail files' function... it certainly saves everything into a nice tidy file... you just can't do anything with it.

Let's try this... I'm going to reassemble my old hardware so I can bring up my old mail client. What would be the best way to save mail from 7 different email addresses, in a way that would allow them to be brought back into a fresh install?

I appreciate your help, I'm about at my wit's end...

If you selected Tools/IET NG/Save all mail files, you would have produces a folder named something like ....default-20200112-1844, and in that folder would be a Mail folder and an ImapMail folder. The Mail folder contains subfolders named after POP accounts and Local Folders, and ImapMail contains subfolders named after IMAP accounts. In each account subfolder there will be mbox files with no extension, such as Inbox, Sent etc., and files named Inbox.msf, Sent.msf etc.

On your W10 computer, in TB select Help/Troubleshooting, click Open Folder to open the profile, close TB. Open the Mail/Local Folders, and copy mbox files from the previous paragraph into that location. Rename the mbox files if necessary to identify them, e.g. InboxGmail, InboxYahoo etc. When you launch TB, you should see the folders under Local Folders in the Folder Pane.

This would be a lot simpler if you just copied the Thunderbird folder from W7 and replaced the Thunderbird folder on the W10 computer:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

I've used my previous method prior, and it worked well, but that was some time ago. As I said, I know TB has changed significantly since then.

I appreciate the explanation... I'll give it a try. If it doesn't work, I'll just reassemble my old hardware and try your linked method... which looks like a lot less trouble. ;)