I would consider a drive that unmounts on its own, without being “ejected,” to have buggy software, and I’d get another drive. Short of that, perhaps scripting an action that periodically uses the shell command “touch” might work (could use Automator, Keyboard Maestro, or a full shell script using cron, etc.).
I’d be very careful about backing up files of any importance or security concerns to Apple–that’s simply not their line of business.