I think a major challenge will be in making sure your external drive is airways given the same drive designation letter. There are ways to make Thunderbird look in a specific place for its data, but there is the rub; you need a fixed location.

You could put your entire profile on the external drive. This would be my choice. Alternatively you could place selected email account folders on the external drive, but splitting a profile in this way makes maintenance more difficult.

I agree with both methods, and the difficulties. The problem I am trying to make reliable is the segregation of archived stuff (projects and conversations I have that need to be curated and those which require frequent attention). Its not just the date, or important photo's of the dog running in the snow; the e-mail includes some business transactions that are documents of events that have a statute of limitations and, some are of modest historical value (to me at least) .

The problem is twofold; the system performance is somewhat burdened with nearly a TB of data stored in my SSD and the inability to structure an archive for the aperodic search by myself and others.

I could likely store these data in a system independent database and cross-reference things, but then managing it using Thunderbird would be extremely cumbersome at the very least.

Back to what is: I believe I can specify the location of the "Archives". But, unless I am missing something, I can't just put it on an arbitrary directory i.e. "D:\Archives" or "\\Server27\archives\".

Your help is appreciated.

Nope. You can move an entire account off to another drive, but not a selected subfolder.

Or, maybe you can, using a symlink or junction. But that's a function of your OS and its filesystem and not specific to Thunderbird. You still have the drive letter challenge. And I couldn't say hand on heart that Thunderbird will handle a UNC pathname correctly.

In the interest of solving my problem, and perhaps helping others, might this be possible: Create an account that has a special name/attribute of "Archive". Place its location on a place of choice: an external drive, a network drive, a cloud store, etc. Something like the "Local" account. Anything tagged for the archive, or filtered to the "archive", would go there, keeping the structure, but located in a better location. I tried to create a "local folder/account" and cannot seem to make it work. Any guidance? Mike

I tried the seperate account route. it still did not fix the drive letters windows insists on using and it does not address Thunderbird need to be able to access all of the profile all of the time.

I think the best long term approach is to export mail to mbox file using the import export tools https://addons.mozilla.org/en-US/thunderbird/addon/importexporttools/

Then import them into another profile and switch between the two as required using the profile switcher add-on. https://addons.mozilla.org/en-US/thunderbird/addon/profileswitcher/

Where would we be with out Paolo Kaosomos and his many add-ons. https://freeshell.de//~kaosmos/index-en.html