Size is no issue if manually copying. I suggest doing a screenshot of the copied profile, showing the folders where content does not appear after the transfer. That would at least be a starting point.

Thanks David - this 'Helsinki' folder is completely empty within Thunderbird - see attached screenshot please: what I labeled 'Level 0' shows you the different Thunderbird profiles below 'roaming' (that I had created after multiple re-installations and trials); then I go all the way down 'Level 1' is the screenshot showing the 'Helsinki' folder among the others; then 'Level 2' goes down and shows you what's within the 'Helsinki' folder; finally, the screenshot which I labeled 'Thunderbird open' shows you that there's nothing within the 'Helsinki' folder when I open the mail program; might I have already screwed up with the labeling/ indexing of the path ('Level 1' screenshot shows 'Thunderbird', 'Thunderbird_old', 'Thunderbird_old2'? THANKS!!!

Which are the contents (screen shot) of the Helsinki.sbd file folder in Local Folders in the Thunderbird profile? Your image shows the contents of a Helsinki.mozmags folder but not of Helsinki.sbd!

thanks - please see attached.

Thanks, Bodo! The "HELSINKI.sbd" folder content shown in the new image is exactly the same as in "HELSINKI.mozmsgs" and it has the same flaw. A folder XYZ in Thunderbird having sub-folders is represented in the profile by an "XYZ.sbd" file folder and an "XYZ" mailbox (or mbox) file (without extension) and an "XYZ.msf" mail summary file. We won't bother about the mail summary file since it doesn't contain messages. Each sub-folder of XYZ in Thunderbird is is represented in the profile by an mbox file (without extension) carrying the same name as the sub-folder in Thunderbird. Your sub-folders of HELSINKI are themselves parent folders for other sub-folders. That means that file folders like Admin.sbd or TRAVEL.sbd contain mbox files of the sub-sub-folders and even other .sbd folders depending of the depth of the folder hierarchy....

Your "HELSINKI.sbd" folder doesn't contain any mbox (mailbox) file which is absolutely necessary for displaying the corresponding sub-folder of HELSINKI in the folder pane. i.e. Admin.sbd should be paired with an (even empty) Admin mbox file without extension, TRAVEL.sbd should be paired with a TRAVEL mbox file , ...etc, etc

To "recover" the sub-folders (make them again visible as sub-folders of HELSINKI) you'll have to create all those missing mbox files manually in a text editor, delete the .txt extension Windows will add and move them in HELSINKI.sbd On TB restart you should find all sub-folders in HELSINKI Don't bother about .mozmsgs folders, they have been created by the Windows search.

Alright, big thank you in advance first of all, and really, if I could get this to work, it would feel to me like a lottery win, seriously (and let me say it plainly again, this is complete new territory to me, and manually doing things in a text editor feels a bit like walking on a rope to me ;-)

Now, I am totally clear on "Your "HELSINKI.sbd" folder doesn't contain any mbox (mailbox) file which is absolutely necessary for displaying the corresponding sub-folder of HELSINKI in the folder pane'."; yet there are two 'but's'; first, this problem (Helsinki folder being empty / no mbox) is also a problem in several other folders (so fixing it manually in this Helsinki folder would only be the beginning); second, I can see some folders that are, when I open Thunderbird on the new machine, showing a partial sub-folder structure, and yet when I go into those sub-folders under PROFILES that actually show up, I still don't see any "mbox" extension files (example of a sub-folder that shows up while others don't: it is appearing with three names, "Marketing Analytics.mozmsgs" (yes, you told me to ignore), "Marketing Analytics" and "Marketing Analytics.msf".

So, your help in (a) understanding where I see those "mbox" files [thus confirming that when I see "Marketing Analytics" without any file extension it actually is the right thing, the mbox file? because you write "i.e. Admin.sbd should be paired with an (even empty) Admin mbox file without extension"] and (b) how to do things via 'one procedure' so that all folders that have this issue of not at all or partially showing sub-folders are fixed simultaneously (and not just 'Helsinki' folder).

Sorry to be so clumsy ... thanks in advance! -bodo.

and yet when I go into those sub-folders under PROFILES that actually show up, I still don't see any "mbox" extension files (example of a sub-folder that shows up while others don't: it is appearing with three names, "Marketing Analytics.mozmsgs" (yes, you told me to ignore), "Marketing Analytics" and "Marketing Analytics.msf". You misunderstood me! There are no files with the extension mbox in the profile. When using Thunderbird's standard mail store format "File per folder (mbox)" it doesn't mean that all those message files would have an extension .mbox. It's the contrary, all those mbox message files have no extension, they are just text files containing the messages of a folder in Thunderbird and having the same name as the folder. An Inbox folder with all its messages is represented in the profile by an Inbox text file, the Sent folder with its messages will be a Sent file in the profile. If you have a folder "Travel" in Thunderbird, there should be a file "Travel" in the profile. "Marketing Analytics" (without extension) also seems to be a mbox file. These are all called mbox files and mbox also is the term for this type of mail store format.

(b) how to do things via 'one procedure' so that all folders that have this issue of not at all or partially showing sub-folders are fixed simultaneously (and not just 'Helsinki' folder). Since all or most of your mbox files have disappeared you would have recreate all of them with a text editor, one after the other, maybe hundreds of them. I am no Windows user and I don't know how to automate this repetitive Sisyphus task.

I still wonder by which mechanism you could have lost all those mbox files when you transferred your profile from your old PC to the new one. You wrote that you did this transfer via copy & paste of the Thunderbird profile folder from the cloud . Maybe the mbox files were already missing in the profile in the cloud? Can you still verify this?

One explanation for the loss of mbox files with a size of 0 kB could be use of an intrusive cleaning tool or certain function of antivirus software. I have seen this several times in another forum when entire folder hierarchies were destroyed because a cleaning tool had removed all 0 kB files. Yet zero kB mbox files in combination with the .sbd folders of the same name are essential for Thunderbird to "ciment" folder hierarchies and display the contents of folders with subfolders.

Oh gee, really thanks for this clarification (once more sorry for my basic understanding) - so yes, "If you have a folder "Travel" in Thunderbird, there should be a file "Travel" in the profile." and now to your core point: "I still wonder by which mechanism you could have lost all those mbox files when you transferred your profile from your old PC to the new one. You wrote that you did this transfer via copy & paste of the Thunderbird profile folder from the cloud . Maybe the mbox files were already missing in the profile in the cloud? Can you still verify this?"

=> please look at the attachment - this is a screenshot from onedrive, i.e. this is how the folder structure looks like for "travel" after I pasted it directly from the previous Thunderbird folders --- clearly, there are sub folders that were saved, yet again (just as for HELSINKI), when I open the new Thunderbird on my new laptop, the TRAVEL folder is completely empty?

So, if mbox message files are "just text files containing the messages of a folder in Thunderbird and having the same name as the folder", then - looking at this screenshot example of TRAVEL - it seems that I actually do have text files in the folders ... yet now I remember something that happened three weeks back, now that you ask about the 'loss of mbox' files: when I tried to copy from the cloud/ my backup on OneDrive to C: on my new laptop, I consistently got this error message (consistently no matter whether I tried to transfer the Thunderbird/ profiles folder by USB or just by copy & paste --- see second attached screenshot - could that be the answer? THANKS !!!!!!!!!!!!!!!!!!!!!

=> please look at the attachment - this is a screenshot from onedrive, i.e. this is how the folder structure looks like for "travel" after I pasted it directly from the previous Thunderbird folders --- clearly, there are sub folders that were saved, yet again (just as for HELSINKI), when I open the new Thunderbird on my new laptop, the TRAVEL folder is completely empty?

The "file folder" TRAVEL.sbd on OneDrive shown in the left image doesn't contain any mbox (text) files such as "2023", "2024", "2025" neither "old travels", absolutely necessary for Thunderbird to be able to display the folders with the same names in Local Folders in the folder pane. This new screen shot is confusing since it shows TRAVEL as a folder in the Local Folders where as in an earlier screen shot a TRAVEL_ folder is a sub-folder of HELSINKI!

So, if mbox message files are "just text files containing the messages of a folder in Thunderbird and having the same name as the folder", then - looking at this screenshot example of TRAVEL - it seems that I actually do have text files in the folders ...

I don't know where you see text files in which files.... I don't see any text files in your image!

yet now I remember something that happened three weeks back, now that you ask about the 'loss of mbox' files: when I tried to copy from the cloud/ my backup on OneDrive to C: on my new laptop, I consistently got this error message (consistently no matter whether I tried to transfer the Thunderbird/ profiles folder by USB or just by copy & paste --- see second attached screenshot - could that be the answer? THANKS !!!!!!!!!!!!!!!!!!!!!

This is an error warning concerning copying (?) a .wdseml file of 0 kB size. All I know is that the Windows search, if activated in Thunderbird, creates a .mozmsgs folder for each indexed folder, and those .mozmsgs folders contain .wdseml files - one file for each indexed message. It is interesting that this warning concerns a file with a 0 kB size (!!) even if in this case it is not an empty mbox file. And actually I won't go so far to presume that all empty mbox files have suffered the same fate ....

Big thanks again! Let me try with more time to digest it better tomorrow - yet just on the last point: so, I just tried again (yesterday) to re-do what I did three weeks back, which was to copy the entire profiles folder from the cloud (onedrive) to an empty folder on C, and yes there is an error ("This is an error warning concerning copying (?) a .wdseml file of 0 kB size.") that would continue presumably hundreds of times if I would not tell the system 'do the same for all similar'; and so I did actually say yes, just do the same ... and that's why I presume that all empty mbox files could be a 'victim' of this copying process ... yet I simply don't know what else to do ;-) THANKS.