I tried to find an add-on that will allow you to move the archived e-mails to another location (disk drive), but unfortunately, there doesn't seem to be such add-on available. For that reason, you will need to manually do such changes.
Here the instructions for Mac OS X 10.8 - both for (1) completely changing the location of the Thunderbird e-mail folder and for (2) moving the archived e-mails only. Before making any change, make sure to completely quit Thunderbird by clicking on the Thunderbird toolbar and selecting Quit Thunderbird.
#1. Change the location of the Thunderbird e-mail folder
By default, Mozilla Thunderbird (all versions) creates a user profile that contains the settings and downloaded e-mails that is stored in this location:
/Users/[your user name]/Library/Thunderbird/Profiles
You can move that folder to another disk drive so that all the stored e-mails and any future e-mail can be stored there. That is recommended if you have another disk drive connected to your Mac most of the time. To make that change, first make a new folder on your second disk drive where you want to store the Thunderbird profile folder. Once you have done that, access the location of the default Thunderbird folder. To do that easily, click on Finder and then on the Go toolbar located at the top, and there click on Go to Folder...
Type ~/Library and click Go. When the Library folder appears, access the Thunderbird folder. You will see several folders and files there, among them is the Profiles folder and profiles.ini file.
You can now copy (move) the entire Profiles folder to another disk drive. When done, you can delete the original Profiles folder. You will then need to tell Thunderbird to look at the different location for loading your Profiles folder (since it has been moved). To do that, open the profiles.ini file with any text editor, such as TextEdit.
You will need to change the last two lines there. For IsRelative=1, set to IsRelative=0. After that, update the Path so that it matches the new location of the Thunderbird Profiles folder.
To do that, right-click (Ctrl-click) the Profiles folder you copied (moved) to the other disk drive select Get Info. The location of that folder will be shown under Where. For example, if it is an USB external disk drive, it should probably say something like /Volumes/USB/Thunderbird
So, if you created a folder on your other disk drive called Thunderbird and copied (moved) the Profiles folder there, that means the full location of the Profiles folder that Thunderbird needs to load would be:
You can now go ahead and use that location as the "Path" in the profiles.ini file. When done, save the changes. Here is an example of the last 2 lines of the profiles.ini file.
XXX is some random name
You can now start Thunderbird and it should work as before, but all the e-mails, both new and archived ones, will be loaded from the other drive, instead of the internal drive.
#2. Moving the archived e-mails only
If you rather want to keep the Thunderbird Profiles folder in its default location, along with all the new e-mails, but only want to move the archived ones to another disk drive, you can follow these instructions.
First, make a new folder on your second disk drive where you want to store the archived e-mails. After that, open the original location of the archived e-mails, which is in:
/Users/[your user name]/Library/Thunderbird/Profiles/[XXX.default]/Mail/Local Folders/
To go there easily, click on Finder and then on the Go toolbar located at the top, and there click on Go to Folder...
Type ~/Library and click Go. When the Library folder appears, access the Thunderbird folder and then access the other folders mentioned here in order to get to the Local Folders
Inside the Local Folders, you should see all e-mail folders that are stored on your computer. Locate the ones that contain the archived e-mails and then copy (move) them to another disk drive. For each archive folder, there are 2 files by the same name, so copy (move) those as well. Once they have been transferred to another disk drive, you can delete the originals to free up the disk space on your internal disk drive.
When you start Thunderbird next time, the archived folders won't appear on the list. To access them, you will need to move the files back to their original location.
I hope this helps!
Should you require further assistance, let me know. If you are afraid of making these kind of changes by yourself, I can remotely connect to your computer and safely apply them for you.