Accumulating messages in Inbox isn't best practice, and now you know why. For the future keep your Inbox as tidy as possible. http://kb.mozillazine.org/Keep_it_working_-_Thunderbird

See this article for the missing messages. Note, this will not necessarily restore them. http://kb.mozillazine.org/Disappearing_mail

To open the Inbox file you can try a decent text editor like Notepad++. https://notepad-plus-plus.org/

Once you either found the missing messages or confirmed that they are gone archive messages in Inbox and compact Inbox.

notepad ++ stiil rejects the inbox file as too large.

I guess 'tidy' has different interpretations; I regularly compact folders and delete non-essentail emails as well as periodically moving interest groups to their own folders.

The current problem seems to have developed after reinstalling XP pro on a new larger HD, downloading the latest version of TB and moving the profiles folder from the old HD. Everything seemed to be fine until I noted the missing 3/2015 files and checked the size of the inbox which had more than doubled since September of this year when the reinstall occurred.

I would like to have an explanation for the random appearance of multiple email copies after I let TB rebuild the inbox index file.

notepad ++ stiil rejects the inbox file as too large.

Another text editor you could try: http://vim.sourceforge.net/download.php According to this information it can open files up to 2 GB. http://stackoverflow.com/questions/12471311/what-is-the-largest-file-size-that-can-be-opened-by-vim-64-bit Note, if you haven't used vi before, this may not be for you.

I guess 'tidy' has different interpretations;

Possibly, but I wouldn't call a 0.85 GB Inbox tidy.

Everything seemed to be fine until I noted the missing 3/2015 files and checked the size of the inbox which had more than doubled since September of this year when the reinstall occurred.

This sounds like your Inbox and/or .msf file(s)s got corrupted. Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

I would like to have an explanation for the random appearance of multiple email copies after I let TB rebuild the inbox index file.

Possibly due to the mail file corruption.

You can always restore lost messages from a recent backup of your profile folder, assuming you took the time to create one. http://kb.mozillazine.org/Thunderbird_:_FAQs_:_Backing_Up_and_Restoring

I have used VI so thanks for that suggestion.

One of the problems has been that I didn't clearly understand TB's file structure or the nature of 'compacting'. This source provided the information I needed (http://kb.mozillazine.org/Recover_messages_from_a_corrupt_folder)

For example, after deleting the contents of a subfolder, I hadn't realized that is was necessary to compact it one more time to actually remove the files marked as deleted.

Haven't resorted to digging through the weekly backups yet but so far, I'm only missing a few emails and rectifying the multiples has proved relatively easy.

If I need to retain some specific email threads over a period of time, I'm now breaking up those subfolders into smaller units. Once in those subfolders, deletion is done only in chronological blocks followed by a compacting operation.

Thank you for the reponses.

More information about compacting. It's recommended to let Thunderbird do that automatically by setting a threshold (e.g. 1 MB). See http://kb.mozillazine.org/Compacting_folders

Also take a look at the Archive feature. https://support.mozilla.org/en-US/kb/archived-messages

When your problem is fixed, can you mark the thread as 'Solved' please? Thank you.