I tried renaming the IMAP INBOX.msf files and...success! Now I will try doing the same with the POP version. The POP emails are stored on the device, although I have Thunderbird configured to leave them on the server until I delete them, which I do fairly often, to keep the web storage fairly clean. I'm hoping I won't lose anything if this procedure is meant for IMAP only.

Aha. When I said success on the two IMAP accounts, that's only on the inbox of those accounts. If I try to open any other folder I get the "unable to open the summary file" popup. What does this tell us? Am I going to have to rename the other folders in those accounts? (I haven't yet had the nerve to rename the inbox on the POP file, the one that really matters.)

Am I going to have to rename the other folders in those accounts?

Yes - rename all the .msf files in every folder. I was hoping you wouldn't have to do that. Actually, you can just delete them.

Thanks hugely. I've just been able to play around a little so far, but I think I can safely say I'll be able to restore the inbox and other folders in all accounts--IMAP and the super-important POP account. But a big problem continues in the POP account: I still can send but not receive, and what I send can't be written to the Sent folder. This was the first issue that started the mess--first I couldn't save to Sent, then everything got hidden. Now, with the .msf files deleted and re-created, it seems in the POP account I can restore what's been hidden but can't write to either the Inbox or Sent folders. Any idea why that would be different from the IMAP accounts? Any idea what to do next?

FWIW, some further thoughts on my continuing "unable to write" problem.

1. I've read some in this community about global-messages-db.sqlite files, TB's search index. But shouldn't that have to do with existing messages, not with the problem of not being able to write to any folder in the POP account? Thanks to having rebuilt the .msf files, I no longer get the "unable to open the summary file" message; my hidden emails are now visible and readable. But incoming emails can't be written to the Inbox. Emails in the process of being composed can't be written to Drafts. I can't delete an email, because nothing can be written to Trash. I can't move an email from one folder to another. Etc.

2. Each time I reopen TB, I have to permit the program to make changes to the device. Why is this happening? Seems to me it ought to indicate something.

3. I first had this "unable to write" problem for several hours, or maybe a day, then "unable to open the summary file" took over and all messages were invisible until I rebuilt the .msf files. Coincidence? Or does "unable to write" lead inevitably to problems with the .msf files? Should I be worried that history will repeat itself? What causes a widespread problem that is resolved by rebuilding the .msf files? What typically causes "unable to write" in the first place?

4. The two IMAP accounts in this profile are OK since I rebuilt their .msf files; only the POP account can't be written to. So I don't think there's a general corruption of the profile. Does this seem correct to others who know a lot more than I know?

What next?

Without uninstalling, try reinstalling TB from https://www.thunderbird.net/en-US/. Your profiles will remain as is.

Will I end up with two Thunderbird apps, or will the new one simply move in and take over, leaving everything as it was: all my messages, settings...?

Will I end up with two Thunderbird apps, or will the new one simply move in and take over, leaving everything as it was: all my messages, settings...?

The latter. Make sure you reinstall the same version.

If it really won't change settings and lose all my stored messages, since I don't know what else to do I guess I'll try it...in the morning.

Thanks for your expertise and your patience.

If it really won't change settings and lose all my stored messages, since I don't know what else to do I guess I'll try it...in the morning. Thanks for your expertise and your patience.

Presuming the initial install wasn't customized, e.g. you installed it to C:\Program Files\Mozilla Thunderbird.

Copy the profile folder and paste it elsewhere just so you have a backup of it.

I've backed up my profile to another location and to a portable hard drive.

I do find C:\Program Files (x86) \ Mozilla Thunderbird, but in Troubleshooting Information the field Profile Folder / Open Folder (local drive) leads to C:\Users\[me]\AppData\Roaming\Thunderbird\Profiles

Is this normal?

Also, further observation: Thunderbird's Activity Manager confirms that there's no problem with my two IMAP accounts but with the key account (POP) "Could not connect to server...; the connection was refused." Not quite the same as "unable to write." Or is it just another way of saying the same thing?

Now, unfortunately, I have to leave town for a week, so this rescue effort will be delayed. When I'm back at it--probably Saturday, 20 December--should I pick up right here or start a new thread?

And meantime, one more big thankyou.

> I do find C:\Program Files (x86) \ Mozilla Thunderbird, but in Troubleshooting Information the field Profile Folder / Open Folder (local drive) leads to C:\Users\[me]\AppData\Roaming\Thunderbird\Profiles

Is this normal?

Yes. I forgot older versions installed in Program Files (x86).

> Not quite the same as "unable to write." Or is it just another way of saying the same thing?

No, not the same. 'Could not connect to server...; the connection was refused.' usually indicates an account configuration problem, but there are several other causes. Come back right here when you return.