Try copying one of the large, non-msf files, the mbox files with no extension, into the Mail/Local Folders location of the profile, while TB is closed, and see if it appears under Local Folders when you restart TB.

Also, select Local Folders in the left pane of Tools/Account Settings, and in the right pane Browse... to the folder on the USB drive, in case it just needs to be reset.

Sorry, maybe not enough information. The large mail folder files are still there but all I have checked only show the last few weeks of emails. It is like TB is no longer reading the whole file to get the email names even though the file size is similar to a backup from October. I did copy a couple of older backed folders to the Local Folders location and allowed them to be renamed and there was no problem with TB building the MSF files and all of the saved emails are listed. It is like something in the last two updates has corrupted these large files. Is there a fix? I tested this on two different PC's and also reverted back to an earlier version of TB but nothing has helped.

I can't explain why your Local Folders are apparently not showing all the messages, as I presume you haven't set the retention to only keep the last few weeks. Folder corruption usually occurs with more dynamic folders, such as Inbox. If you have antivirus apps scanning mail folders, that is a possible cause of corruption; add the TB profile folder, and the LF on the USB, as exclusions or exceptions.

No, nothing new. Like I said, this has worked fine for years and now after the latest updates it appears that all are missing emails. My only antivirus app is Windows Defender which has never caused a problem and there was already an exception for Thunderbird. I can still open the TB files in notepad and they don't appear to be corrupt but I don't know what the rules are for the internal structure to see if something is missing between emails or .... I was hoping that someone from Mozilla would be able to help as these emails are extremely important and project related and I need to have this fixed asap.

The problem may be due to your Local Folders not being in the standard location, and perhaps the new TB, which is particular about profile folders, has a bug. You may wish to report it on Bugzilla. But in the meantime, it appears you can recover all the messages by copying the mbox file into Local Folders in the standard location.

No idea what you mean by mbox file. If you mean the main folder files with no extension, moving them does nothing. They still do not show all the emails inside the files and some even show no files when there are hundreds of emails in the file. Is there an app or program that can scan these bulk email files and possibly fix the problem?

There are several free mbox to eml converters. Whether they will handle an mbox that is possibly corrupted is unknown.

So there was no fix or repair for the problem caused by Thudrebirds most recent update. I lost all of my stored business emails archive from the last 3 months. Gone. Unbelievable.

The latest updates or any update would not have touched those files. A Thunderbird update does not access USB's or mbox files.

re:the main folder files with no extension... I can still open the TB files in notepad and they don't appear to be corrupt Check to see if emails have been marked as deleted and hidden. So they may appear in the mbox file, but not in Thunderbird.

Exit Thunderbird. Use Notepad or suitable text editor program to open mbox files.

Emails are written to the file one after the other, so oldest will be at the top. Each email will start with these lines: this is an example

• From - Sun Dec 28 18:14:40 2014
• X-Mozilla-Status: 0001
• X-Mozilla-Status2: 00800000

Use 'Edit' > 'Find' Start at the top, look for this line: X-Mozilla-Status: (not X-Mozilla-Status2:)

• Make sure each X-Mozilla-Status: has the number 0001
• edit the number as required working down through the document.
• Save the file.
• delete the same name '.msf' file. A new one will be auto created.
• repeat for each mbox file effected.

Restart Thunderbird.

To test the default location is working.....did you do this:

Exit Thunderbird first. Copy the 'Local Folders' folder stored on USB

Access the profile default location: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ Mail folder

Paste the copied 'Local Folders' into 'Mail' folder. If you choose to keep the original then let it name it as 'Local Folders-1'

Start Thunderbird. Menu app icon > Options > Account Settings > Local Folders Check the 'Local directory:' for 'Local Folders' by clicking on 'Browse' to make sure it really is pointing to the added 'Local Folders' or 'Local Folders-1' in default location. click on OK Restart Thunderbird.

re :I have had a 64GB USB set up as my Local Folder for years.... I lost all of my stored business emails archive from the last 3 months.

So these are very important documents, in which case is there a backup? Maybe copies still on server which you can access via webmail ?