Below -18°C.

I use a probe thermometer to verify. Mine hovers between -20°C to -22°C

Until you try to sort your log files alphabetically.

Use a modern filesystem that manages file integrity for you.

I use btrfs.

Don’t forget to scrub and checksum your drive before making backups. You don’t want to copy over rotten bits.

Same goes for the backup. And the backup’s backup.