https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

Is the profile big because of IMAP or POP accounts? If it's big because of IMAP accounts, you can exit thunderbird, locate the imapmail\<IMAPacccount> folder and delete the message folders. That should reduce the profile to more manageable size. The only reason I can think of that you would not be able to copy any file would be that Thunderbird was still running, I don't grasp the 2-point security, but turn it off. Now, try again and let me know. To locate the above-mentioned IMAP folder, do this: - click help>troubleshootinginformation - scroll to 'profile folder' and click 'open folder' - click the imapmail folder - exit thunderbird - delete the message folders - NOW, create the profile copy: here are reminder steps Here is the link for moving a profile in Windows when installed in default location: https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer here is the info in brief steps: # ensure thunderbird is NOT running on old machine # enter %appdata% in search window on task bar and press Enter key # this should open windows file explorer in Appdata\roaming folder # highlight the Thunderbird folder and copy to external media # ensure thunderbird is installed on second computer, started once and NOT running # enter %appdata% in search window on task bar and press Enter key # you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it # start thunderbird and all should be there