If there's a hardware or filesystem problem that causes an error, the default in TC is "remount-ro" (which I've seen before with faulty drives and USB connections), so the filesystem becomes read-only but is still mounted. You can change this behaviour in /etc/fstab or with the mount command so the errors are ignored ("errors=continue"), but I wouldn't recommend it since the filesystem might get so messed up that TC won't boot anymore (although that might also happen eventually with the default setting).