The email client Mozilla Thunderbird uses a similar structure of storing data in so called profiles which are saved by default in the user directory of the operating system.There are a number of reasons why some users might want to move the data from the Thunderbird folder to another custom folder. Some that instantly come to mind are running out of storage on the default hard drive, moving the profile to a hard drive that promises speed improvements or moving it to an encrypted folder on another hard drive for security reasons.
Whatever it may be it turns out to be very, very easy to move a single profile or even all profiles from the default location to another one. The first step is to close the Thunderbird email client so that it will not write data to the profile folder anymore. The default location of the Thunderbird profile folder depends on the operating system in use, below is a list of common locations:
- Windows XP/2000, %AppData%Thunderbird
- Windows 95/98/Me, WINDOWS\Application Data\Thunderbird
- Linux, ~/.thunderbird/
- Mac OS X, ~/Library/Application Support/Thunderbird/
Copy one or several of the Thunderbird profiles to the new location. Now open up the file profiles.ini in the original folder and edit the path pointing to the new profile folder locations accordingly. You also need to change the parameter IsRelative=1 to IsRelative=0. Please note that this is valid for all profiles which means that you need to add the full path (e.g. c:\thunderbird\profiles\profile.code\ in the Windows operating system) to profiles that are kept in the default location. Start the Thunderbird email client. If everything worked well it should display the emails and other data as usual.

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If your setting up Thunderbird for the first time then it is best to use the profile manager to create a profile & set a name & place(on a non-system HD) to store your profile & then you can back up the folder everyday to another media if needed & if you need to reinstall just use the profile manager to point to the profile location to all should be good & the same works with Firefox.
Its how I do it & never have had a problem.
Can you just copy the address book and email, say from your desktop to a portable version on a thumb drive, without copying the message rules and server settings?
I would like to do something like that, so that I am current all the time, but leave the messages on the server with the portable version. Then synch the address book when I get back to the desktop.
I never tried moving only the emails. You could try something like mozbackup to only backup the emails and restore them to the USB device. Should be possible.