The difficulty with this kind of question is what do you want from, or wish to do with the data you are planning to "archive"? And is it truly an "archive" or more a "backup"? The difference being that you can search an archive but a backup is for repair after a breakdown or accident. A challenge with an external drive is that it won't appear in the same place every time it is plugged in. In Windows that means the drive letter assigned to it may change, depending on which slot you plug it into and what else may have been plugged in already. In Thunderbird can can nominate the location of an account's folder, so you could put your entire Local Folders account onto your external drive, and tweak it to make archive folders there. But you'd have to nominate a location and as described above, that can be awkward if drive letters are involved. An alternative is to export the messages, creating sets of folders and messages in them as regular folders that you can store anywhere, such as on your external drive. So now you have messages stored outside the email client and you read them with whatever tools are appropriate to the format used. They could be plain text, html or pdf for instance. You could store them as eml files which will open in Thunderbird. However, regardless of the format, they will now no longer be threaded, so following a chain of messages making up a conversation will be harder to do. And you won't have Thunderbird's search tools, so your searching experience will, in general, be less precise. If you want to try the export route, do look at the ImportExportTools add-on. https://addons.mozilla.org/en-GB/thunderbird/addon/importexporttools/