Hi Thomas,

There are several ways to accomplish what you need. Here are two:

1. Put your messages in local folders in Thunderbird if they are not there already. In the file system, copy the local folders to an external drive or two. If you are like most users, the messages will be in mbox format. You will be able to import them into Thunderbird and other e-mail clients. You could also read them in apps that do nothing but display mbox messages. Then delete the messages in Thunderbird.
2. Get the excellent add-on ImportExportToolsNG and export your messages to an external drive in your choice of six formats. Then delete the messages in Thunderbird.

I have many files of mbox messages stored outside Thunderbird on my internal drive and on my back-up drives. These days, when I export messages from Thunderbird, I use the eml format because it is more efficient, maybe less prone to errors, and easy to import into another client that I use occasionally. MBOX may be more common, so you may want to use it.

When I do these things, I do not bother to compress (zip) the files because there is plenty of room on my drives and I don't want to have to de-compress before accessing the messages again. You can compress if you want to.

Please ask if you need more details.

This is sounding good, but let me sharpen my specific case. In Thunderbird and in the file system I have a directory Z_History (.sbd in the file system). This is 14.6GB on a 256GB drive, so significant. I have made no meaningful use of this data in years. It includes data to 1998. It is organized in three rough logical blocks: 1) a dozen topic-specific directories for old projects; 2) fourteen directories corresponding to 14 years of archive; and 3) a directory below which are all the pre-2010 folders by year.

I doubt I will ever need to access any of it at this point, but of course, I doubt want to lose the option. So, how do the .sbd folders and .msf files work? Is this structure in Tbird or does it just read what is on disk and display that structure? I.e. Could I erase everything under Z_History and then, if I needed it, copy the relevant .sbd, .msf, and mail files back to the main disk, restart TBird, and go to town? I don't fancy starting with a whole new tool. Would I need any more than the path to the mail file(s) I needed?

These are all local folders, not stored on the server, right?

Could I erase everything under Z_History and then, if I needed it, copy the relevant .sbd, .msf, and mail files back to the main disk, restart TBird, and go to town?

Yes, if Z_History was saved outside Thunderbird. But copy/move/delete Z_History, not just what is within it.

The .msf files are index files used only by Thunderbird. They will not be used if you import the messages into another client. They are also not necessary for future use in Thunderbird because Thunderbird re-creates them as needed. .sbd files are important for maintaining the directory structure.

The simplest approach is just to copy Z_History in the file system to your back-up location, then delete it from your profile, and re-start Thunderbird. (As long as they are local folders. I am not certain that there would not be complications if you took this approach with server folders.)

As long as Z_History is saved, you can move it back into your profile for use in Thunderbird or you could import individual mbox files (the ones without an extension) into Thunderbird or into a different e-mail client.

I have seen situations in which moving such folders back into Thunderbird did not work even though it should have and usually does. I do not know why this process fails sometimes. it may have been because of missing .sbd files. If it fails, that add-on ImportExportToolsNG should do what you need.

For extra flexibility, you could export folders in eml format in addition to copying/moving Z_History. I do not kmow if there are any popular e-mail clients that import eml files but not mbox files.

Is this structure in Tbird or does it just read what is on disk and display that structure?

I am not sure that I understand. You can see this same structure in Thunderbird and in your Thunderbird profile in the file system. So, yes, it is in Thunderbird. And yes, Thunderbird does read it on disk so it can display it within Thunderbird.

I do not know with certainty, but it may be good to compact the folders before removing them. Unfortunately, the compact process causes data loss in rare situations. So back up, compact, check for data loss, then do this operation?

This is all on my local desktop. There is a Linux server which is the source and destination of the mail, but plays no other role (and which we would love to replace with an external managed service, but that is a different question). I have already copied Z_History.sbd to my D: drive ... just straight OS copy. I understand that if I now delete Z_History.sbd and everything under it from my C: drive using regular OS commands and restart TBird that part of the structure will just be gone. And, should I develop some passion for some or all of it, I will simply need to copy Z_History.sbd and any relevant directories and files below it to Local Folders, restart TBird, and presto.

Where would I find ImportExportToolsNG?

The question you were unsure of was just about whether TBird figured out the directory structure on start up based on the .sbd files or if it had a binary version internally ... which you have resolved.

Where would I find ImportExportToolsNG?

Tools > Add-ons and Themes > Find more add-ons

OK, but i would rather have the other questions answered.

Which? I see no unanswered questions.

Just that my description of removing and re-adding Z_History mail is accurate.

Yes, it is. But I would back up your whole profile first. Strange things can happen.