I cannot work out how to save email messages in Thunderbird to a separate volume I have almost 500000 messages and my home hard drive cant handle that much

I can think of a few things that might be appropriate, depending on your exact goal... - With the aid of the "ImportExportTools NG" add-on, you can export messages to a file that can then be archived anywhere you like. The messages won't be immediately accessible, but it's easy to import them back into a Local Folder for use. - You can move the ENTIRE Local Folders store to any other location. This article describes how. I have not tried it myself. You probably want to see the " For accounts not using the Global Inbox" section: https://kb.mozillazine.org/Moving_your_mail_storage_location_-_Thunderbird - You can move your ENTIRE Profile to another location. This may not be what you want, but it's a possibility. https://kb.mozillazine.org/Moving_your_profile_folder_-_Thunderbird - I've seen this add-on being mentioned as to being able to add new Local Folders on other locations. I have not looked at it closely, but maybe it will do what you're looking for: https://addons.thunderbird.net/en-us/thunderbird/addon/localfolder/