Tell us what you have first. Is this just the profile, or is it the Thunderbird folder that includes more data? Did you install thunderbird from https://www.thunderbird.net/en-US/thunderbird/all/ ? If you have the Thunderbird folder, the following steps work:

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:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. 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
8. start thunderbird and all should be there

But if what you have is the profile, the steps are different. Also, if the profile was copied while Thunderbird was running, it may be corrupt.

Thank you for your reply.

I copied c:\users\sterr\appdata\roaming\thunderbird to a USB stick from my old PC with TB closed.

On my new PC I closed TB, which I had downloaded from https://www.thunderbird.net/en-US/thunderbird/all/

I then copied the saved information on USB to c:\users\sterr\appdata\roaming\thunderbird. There are no prompt to overwrite.

Two things that I noticed. 1 - on coping data on the old PC it had a problem at the very end with files, prefs-3.js 5708KB and prefs-5.js 0KB on both occasion I pressed YES to the question.

2 - The two versions of TB are slightly different but I cannot see a way change them as both versions state they are upto date.

     The old PC TB version is 140.1esr(64-bit)
     The new PC TB version is 140.1(64-bit)

I hope this answers your questions.

there no prompts to overwrite because you didn't copy properly. copy to the appdata\roaming folder, not the appdata\roaming\thunderbird folder. If you look, you will see that your copy is at c:\users\<yourid>\appdata\roaming\thunderbird\thunderbird

Comment: there should never be other than prefs.js - the appears of more than than indicates there either was a problem in the past, or there is one now. We'll know more after you finish the copy and restart.

Thank you very much for you help. It appears to have now transferred all the data. The only excuse I can give is that I have spent too long in setting up my new PC! Thanks again, Stephen

I am pleased that all is ok. Save this email and the next time, your transfer will take just a few minutes. :)