I suggest saving to draft instead. saving to file has higher overhead.

Thank you David for your reply. Normally my Thunderbird does indeed save as draft. I don't use Save as file.

My auto-saves and Ctrl-S (while writing a message) go to Drafts folder, except on intermittent occasions when Thunderbird hangs after Ctrl-S.

It seems the save to draft process fails occasionally - any tips how to troubleshoot this?

Troubleshooting is a real can of worms. but my first suggestion to everyone is to look at what else is monitoring the disk for updates and eliminate that. Things like streaming backups, cloud sync of the storage location and antivirus all have a role to play normally in accessing your disk.

Generally Thunderbird stored your profile in ah hidden location accessed by the windows variable %adddata% some folk routinely change this location for various reasons and they frequently have issues with Third party applications that do not recognize the Thunderbird mail store as application data after the move.

Thank you Matt, I don't use streaming backup and cloud sync, and intensive access of my local disks would seldom happen. I wonder if there's an occasional glitch with Thunderbird writing a Draft to the mail server (remote) that isn't being handled properly?

Try Windows started in Safe Mode. Easier first step than try to guess whether something in Windows is impacting Thunderbird.

Thank you Wayne, I understand that's a good strategy in many cases.

However, my problem (see OP) occurs intermittently (day/weeks between occurrences) and it cannot be reproduced at will. Even if I ran Safe Mode all day (not feasible for real work) I would still not know whether safe mode eliminated the problem or whether the problem was going to recur a bit later.

Thanks to all respondents in this thread. Your comments helped to focus my thinking :)

Instead of Drafts on the remote mail server, I have now set Thunderbird to use only Drafts folder on my local disk. So I will continue normal work in normal mode and see whether the problem still recurs... Fingers x'd.