Probably you didn't copy the profiles.ini file.

You're right. I've located the file, and can copy it to the profiles folder. Since my custom data folder is different now, do I need to change the path inside the .ini files? Currently it reads:

[General] StartWithLastProfile=1

[Profile0] Name=default-1457898911166 IsRelative=1 Path=Profiles/tl0mne1o.default-1457898911166-1518458288817 Default=1

thanks

Yes, the Path must point to your profile folder.

Does the profiles.ini go into the data folder? Where does this file belong?

The profiles.ini file should be in the Thunderbird folder, which has a Profiles subfolder and other items. The Profiles folder contains subfolders which are the 'profile folders', e.g. a folder named ab12cd34efg.default.

A typical file structure is: C:\Users\username\AppData\Roaming\Thunderbird\Profiles\ ab12cd34efg.default

Moving data from one computer to another is a lot easier if you transfer the entire Thunderbird folder.

More confused than ever...I have installed TB and created a profile that points to my data folder, which I transferred to the new PC from the old one. This data folder has emails, attachments, etc, and it also has a ab12cd34efg.default folder. But nowhere do I see a profile.ini folder or file. I have copied the ini file from the old PC, but cannot find where it is supposed to go on the PC.

Does it goes in the data folder? If so, where? Does it go in the C:\Users\username\AppData\Roaming\Thunderbird\Profiles\ folder?

You wrote: 'Moving data from one computer to another is a lot easier if you transfer the entire Thunderbird folder.' That's exactly what I did: I installed TB program on the new machine, the copied my data folder from the old to the new, yet when I start TB and specify the new profile that points to the data folder, TB thinks I am opening an account for the first time.

thanks,

Not sure what you mean by 'data folder'. The emails etc. are not in the same folder as the ab12cd34efg.default folder - the latter contains the emails etc.

On the old computer, copy the Thunderbird folder, C:\Users\username\AppData\Roaming\Thunderbird, to a USB drive, DVD or external hard drive. On the new computer where TB is installed, delete the existing (mainly empty) Thunderbird folder while TB is closed. Copy the Thunderbird folder from the USB drive and paste it in the same place where you deleted the existing one. When you start TB, it will recognize the data from the old computer.

This is very helpful, thanks, but I'm not sure what thismeans:

Not sure what you mean by 'data folder'. The emails etc. are not in the same folder as the ab12cd34efg.default folder - the latter contains the emails etc.

What do you mean by "the latter"? thanks,

The ab12cd34efg.default folder contains the mail and address book files; ab12cd34efg.default and the mail and address books are not located together in the same containing folder.

In a sentence with A and B, A is the 'former' and B is the 'latter'.

thank you!!

Sorry, but now there's another issue: I've migrated TB to the new PC, but now I see loads of duplicate emails. These emails are in the inbox, but since I'd already archived them they are also in the archive folder.

Should I delete the dupes? Archive them as well? I don't know why this happened...

thanks

Hard for me to say how you got duplicates, but as long as you back up your profile (or better, the Thunderbird folder), I see no reason why you shouldn't delete them.