I appreciate the amount of info you've provided, but what I find missing is the circumstances under which this happens… Is it when you receive mail, when you try to move messages to some specific folder, or what? And what type of account is this (POP, IMAP)?

Sorry not to have included that... but Yahoo is my email 'provider', and I can access my messages on their server via mail.yahoo.com and the messages are there. I've been downloading them from Yahoo to Thunderbird for more than a year without problem... and when I tried to do that a few days ago, the "Can't write..." error popped up after it logged into my account and began downloading. This is a POP account. I am able to send mail from Thunderbird, just not able to get it from Yahoo. No other 'events' (upgrades or problems) have happened recently. Sure appreciate any suggestions you might have.

OK, so this is the Inbox of your POP account that cannot be written to when checking for new mail…

What about moving mail out of there? Do you keep all your mail stored in a single huge Inbox? Try creating one or more folders under Local Folders and moving some or all of your mail there. Do you get any error messages as a result of doing that?

If not, move all your mail out of Inbox and into other folders, then right click on Inbox and choose Compact. Do you get any errors when trying to do that? If not, and you may successfully compact it, does that solve the problem?