If you stick with Thunderbird,
This backup tool is a must have. A profile (including all messages and settings) can be easily made, and stored anywhere. Flash drive, another partition, wherever you want.
The same program can be used to restore a backup in case of Bad Things Happening. (Basically restores Thunderbird to the day you made the backup. Subsequent messages will be downloaded again as new mails, if they haven't been deleted from the server.)
I do it every week. More often if there are very important emails that I want a backup of.