There are many blog posts on encrypting backups, but none works for local drives.

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#How to

• Encrypt the external backup drive using TrueCrypt Show archive.org snapshot or PGP Show archive.org snapshot or similar.
• Mount it. If possible, let this happen automatically.
• Tell Time Machine to use it for backup.

#What is NOT working

• Backing up to disk images as described in this blog post Show archive.org snapshot . Apparently, sparsebundle images & co. are only supported for afp:// access. For local backup, Time Machine does only recognize volumes and partitions.