I've left a comment upstream asking if this has been fixed recently in the Thunderbird 3.0 beta. I've been trying to reproduce this, but Shredder (Thunderbird 3.0 beta4pre) behaves as expected, refusing to download messages if there isn't enough disk space, only downloading new messages when disk space is freed, and not corrupting existing messages in the meantime.
I've left a comment upstream asking if this has been fixed recently in the Thunderbird 3.0 beta. I've been trying to reproduce this, but Shredder (Thunderbird 3.0 beta4pre) behaves as expected, refusing to download messages if there isn't enough disk space, only downloading new messages when disk space is freed, and not corrupting existing messages in the meantime.