I don't know why a symbolic link should give trouble. We do know that NAS users tend to have problems. I don't know why this is, but it gives an uneasy feeling that, as far as Thunderbird is concerned, any drive that isn't immediately and constantly online and available can fall short of expectations.

I was annoyed recently to get a new machine with both SSD and HDD and to find that there is no easy way to get Windows to automatically put all the user stuff onto the HDD. By default, it would be still filling up my precious SSD. (This is trivial to do properly in Linux; just set the mount point for /home on the appropriate device.)

For Windows, I went about it by moving individual parts of my personal data stores (Document, Pictures, Music etc) to the HDD. I was thinking about moving the whole of Users to the HDD. but have read many warnings that doing this will break Windows updates. On that basis, I wasn't sure there was any clear dividing line where I could use a symlink without adversely affecting Windows and its byzantine systems. In particular, it seems to me that AppData should definitely be somewhere safe, and easy to isolate and backup.

You don't need to do any of this with Thunderbird. You can put the data store part of your profile wherever you want it, and adjust profiles.ini to point at it. However, this will break your data separation paradigm, so I appreciate it isn't a "clean" answer.

Thanks for your tip - very useful. Sorry I've been distracted by other stuff blowing up so couldn't reply sooner - in fact my situation is exactly like yours and I guess many others. Like you, I want my 'stuff' in one place to backup, and not on the precious SSD. I'm just surprised symlinks decided to stop working now - presumably bc win10 broke something that tbird relies on?

Win10 is just adding chaos to chaos. So I also want to be able to police system folders for cache, temp and other pointless system and app repositories, local, roaming etc. Other features are the death of security compliance - such as 'last used' lists, auto-save folders, search indexes, spellchecker and 'system volume information'...

The fact they can't decide how to arrange explorer so you can actually see D: off the bat, and yet the (always empty) media card slots are right there! Under 'user' I now have two versions of desktop, document and downloads... user is under desktop - and desktop is ALSO under user, its insane. D: is under 'this pc', and so is documents etc... and yet 'this pc' is ALSO under desktop. wut?? The only sensible feature is 'quick access'... which means you can bypass the train-wreck altogether. imho thats why everyone now 'needs' cortana and search - bc finding their stuff is so confusing....

Also Win10 (on my desktop which win10 clearly still thinks is a phone) hijacked an old contacts list it found in a backup folder without asking me, wants to offer me a camera that the pc doesn't have (and yet has the most crass scanning app with not even a brightness control) and clutters the start bar with endless bloat apps that i literally never use bc i have my own (like tbird, firefox etc). But MS won't let me uninstall these (even on win10 pro). And every upgrade, MS asks me AGAIN whether I want to keep using (say) tbird instead of <some MS software product>.

Last completely offtopic snippet - I tried ONE 'app' from the market - the built in clock timer - and in the first use it got stuck in a loop and I couldn't cancel the alarm without restarting the PC! (Actually I lie - one app i was pleased to use was paint3D.)

Oops, all aboard the win10 rant train, woo woo!

OK, long time but I fixed it... by following advice on forums and Mozilla knowledge base - basically learning about profiles, mail storage...

Then editing profile.ini, deleting spurious .lock file, using cmd line (to reach profile manager), setting up a new profile folder (twice), copying the old profile across...

then i was going to repair each account inbox, but it turns out bizarrely some items were read-only, so i made the entire profile 'writable' in explorer...

All of which shenanigans defeated the errors one by one - - completely blank profile (from update weeks ago) as if it was a fresh install. - 'profile missing' error message - still no emails! - 'unable to open summary file' error message.

So now it is all back to normal.

The question remains as to why this occurred. i.e. Thunderbird update or Win10 update. No clues but I did have a symlink from C to D and the update did not obey this so...

On the way i discovered that windows keeps an entire cache on C for their own nefarious reasons. But today I don't have time to solve that.

I also couldn't fix my symlink from C to D (again from a Win10 gotcha), so lucky for me Thunderbird at least has the option to move the profile folder somewhere sensible.

Bonus - today I found a bug in this very support site at mozilla.org - the link from my user account didn't link to this question at all...

"But I have patience and experience. A man can do anything if he has those."