Compacting only deletes messages that you have previously clicked to delete, freeing up space in the folder.

David, Thanks for your speedy reply. A bit confused how you deliberately "delete messages that you have previously clicked to delete"? That's sorta why I want to do this. I want some but not all messages; mostly from friends or businesses. Thanks

In mbox files, a deleted message remains, but is marked for delete. That means that it still is taking up space on your computer but is not accessible. As deleted messages increase, this increases the size of the folder. The process of compacting is removing the 'dead' messages from the folder. This is just basic disc space maintenance.

David, In mbox files, a deleted message remains, but is marked for delete. Glad you are knowledgeable. A bit above my skis tho. I understand most of it.

Can 'dead' messages be saved somehow to a USB or elsewhere?

I'm not looking to compact them or remove them or delete them. Can I copy them to a USB w/out saving them individually esp the business ones and friends?

Thanks

If you plan to save them, why delete them? Instead, you could move to another folder. Or, you could review the trash folder prior to deleting, and move selected messages to another folder. Maybe I am not understanding the question.

David, If you plan to save them, why delete them? Just trying to compact folders to save space.

I use Msg Filters to organize my emails. Which seems a good thing, but I'd like to flag certain folders or files for safekeeping so to speak.

Not sure what you mean by "review the trash folder prior to deleting".

I'm compacting to reduce the size my emails take up. Maybe I'm confusing it w/gmail when I get low on space.

How much is my TB emails taking up on C: drive anyway? How can I check?

I'm not looking to compact them or remove them or delete them. Can I copy them to a USB w/out saving them individually esp the business ones and friends?

See David's answer to that exact question asked by you here https://support.mozilla.org/en-US/questions/1580139#answer-1816177

I suggest you explore the import export tools addon and have a look at it's features and forget compact. It is a housekeeping function. Nothing more and destined to a place in history when the new database comes out in a few months. The fact that deleting a message is a two stage process appears to confuse people and they see it as something like zipping or compressing the data. it is not, just allow it to happen as it is necessary and ignore it beyond that.

You can get a broad brush view of your email storage by these steps: - locate c:\users\<yourid>\appdata\roaming folder - highlight the thunderbird folder, rightclick, and click 'properties' - for detail, you would need to enter the profile, click on each account, and the check folder sizes.