Each time the files are updated, the date/time changes. From your description of your backup strategy, you will end up with many copies. You might consider backing up once a day to minimize the multiple copies. This is not a Thunderbird issue, but a result of how you do backups.

Thanks for your response. In fact, I do back up only once per day and even with this back up schedule my flash drive fills quickly. I agree it doesn't seem to be a TBird issue but I'm trying to cover all possibilities. For unknown reasons only my Gmail folders are copied each day and backed up to the flash drive as if each day's files were not connected to the previous day. This does not happen with any other folders and files so it seems there is something unique taking place. I just don't know what it might be. I thought maybe someone else also ran into this problem.

I use "Backup Using File History".

That implicitly says a copy of every version of every file. When you get an email the file changes. When you delete an email it changes again. I would see that strategy potentially making dozens of copies of the files daily really.

Just to demonstrate. Open the window file explorer. Locate the file, right click it, select properties and have a look at the previous versions tab. That is what you are selecting to backup.

Personally I do not have the windows shadow copy enabled at all as it uses a very significant amounts of disk space, and in the case of my 4.5Gb inbox file basically stops the system while it is copied after every change.

I do not allow the windows indexing service to index those files in my profile folder either as yet again in slows the system. Indexing huge files that have had maybe 200 characters changed is not a good use of the speed I bought.

Drop the history strategy and the problem will go away is my take on it.