What happens if you create a subfolder under Local Folders (call it Test), and select an eml file on the USB drive in File Explorer, then drag and drop the file onto the Test folder? If this is successful, you can repeat with a selection of multiple eml files and once in Local Folders, move the messages to other folders if necessary.

Brilliant! That seems to be working......It will take a while but I'll crack on and let you know how it goes. By the way, how is it that it can read the files when they are still eml files? David

Now I am back from work... 2 weeks on 2 weeks off.

And referring to the topic I finally have time to respond to https://support.mozilla.org/en-US/questions/1113948

Install this add-on https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Instruction for installing an add-on here https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Select your test folder, or whatever folder you want to use, right click, select the import/export option and then import message from a directory. Navigate to where your EML files are located and import them all.

That is basically what it says in the KB article I think.

Hi Matt Welcome back......I guessed you had been away for some reason. Your latest post is what I did originally.......installed the add on and used that to import from the USB........but it did not work........it just installed the folder structure but no files. I tried it twice with the same result. I am currently trying the manual method suggested by fhowes......that seems to work except where I try and copy in more than about 700 files at once.......but it is going to take several long evenings to do it. There seems to be a volume problem that made the add-on not work, and is also causing me problems when I try and copy them in manually? I am copying them in through the Thunderbird screen folder structure. Perhaps I should be copying them in directly to the hard disk somewhere instead? David

I just tried the add-on method a third time with the same result.....pulls in the folder structure but no files.........only takes about 15 seconds which I think is too quickly to pull in all 3GB of files as well?

you might be onto something with the number of the files. The add-on might not cope with more than 30 thousand is my guess.

However. Open the error console on the tools menu, Clear it and then report what errors appear when you try that import that does not work. Then at least we will have whatever error messages the add-onm issues (If any that is)

Given your using Windows 10. Ensure defender is tuned off before you start, Do you have any other anti virus that might block the rapid opening of files? after all opening and moving tens of thousands of files might liike like one of those encryption malwares to the anti virus.

I have Norton Security so I turned that off. This time it stopped before even copying all the folders. The error message is:

Timestamp: 23/03/2016 23:06:16 Error: : Component returned failure code: 0x8055000c [nsIMsgFolder.createSubfolder] Source File: chrome://mboximport/content/mboximport.js Line: 946

It is possible that the usb file is now corrupted from trying to move data across manually, so I could try and create a new export from the old PC but unfortunately that PC is now packed away and I would have to unpack it all and set it up again which would take some considerable time. I am on UK time, so that would have to wait until tomorrow. Let me know what you think I should do, and I'll try it tomorrow. Thanks

I have not looked at the source, but do you have folder names with / or \ in them? I think that when might be happening is the add-on thinks a folder is a sub folder because of the existence of those in a folder name. (I has been a long running javascript issue)

Restarting Thunderbird between attempts might also help as your removing and adding folder with the same name between attempts. If might just be getting stuck on something it thinks it remembers. (old cached information)

Hi Matt I don't think I have any folder names with / or \.........however I have noticed that the exported file from my old PC onto the USB has changed some of the folder names. For example a folder that was called "Aradon Forwarded" has become "Aradon forw 822" I tried again doing the import having restarted the computer and turned off Norton but it terminated half way through creating the folder structure again and I still got the same error message:

Timestamp: 24/03/2016 12:30:20 Error: : Component returned failure code: 0x8055000c [nsIMsgFolder.createSubfolder] Source File: chrome://mboximport/content/mboximport.js Line: 946

Other factors that I mention just in case you think that they may be relevant: 1.........I now have the folder structure three times in Thunderbird......all with different parent folders to keep them distinct. The first I have kept because I already manually copied in lots of files using the method suggested by fhowes and did not want to have to repeat that if other solutions don't work, but I created the folders structure for this manually and it is not complete. The second I have kept because it has the complete folder structure and it will be much quicker if I continue with the fhowes method because I don't have to create the folders manually before I copy in the files. The third is incomplete and is the one I have just tried to import..... I will delete that one. 2. Some of the folders/files exported from Windows Mail were originally imported into Windows Mail from an older machine that had Outlook Express (although there never seemed to be a problem with them in Windows Mail)

If all else fails and I continue copying in files as per fhowes I have a couple of questions: 1. Once I have copied them into Thunderbird how does Thunderbird read the files and preview them even though they are eml and not Thunderbird formats? 2. If I ever (heaven forbid) had to move all the files again from Thunderbird to another system, would it get lost trying to export them to the new system because half of them are eml format and half Thunderbird format? 3. I notice that each folder on the USB has an extra .FOL file .......do I need to also copy that file into Thunderbird if I am using fhowes method?

What do you recommend that I try next?!

Eml is a standard format that Thunderbird itself uses to export (and import) messages. I imagine that as they are added into a folder in Thunderbird, they will in turn be added to the underlying mail store - usually a monolithic mbox file.

When I tried copying a Thunderbird profile into a new Windows 10 machine, it too had a problem copying nested folders. On the first two attempts, it just copied the top and next layer down, but no files. On the third attempt (maybe the gods reward perseverance) it all copied over. Like you, I was startled at unfeasibly short copy times - but there were no error messages to say that it had encountered a problem.

I think Windows 10 is buggy. Note I was using Windows 10's own tools (File Manager aka Windows Explorer) not any Thunderbird add-on.

Your .fol files appear to be specific to windows mail client: http://www.file-extensions.org/fol-file-extension

Thunderbird won't need (nor have any use for) these files.

Thanks Zenos, that helps me to understand it a bit better. David

Thank you guys for your help on this. However, I never managed to successfully import or copy in the old messages. I have now installed a different client, and that imported them all straight away without a problem, so I am using that.