http://zqktlwk5ilgohy63eyrptyb4mc76bunfmxcnstkmveyjnskxkpq3b5yd.onion/wiki/Intrusive_Surveillance
Note that if your system is running remotely, it may not be too happy about brining sshd up for you after the unmount or upon bootup, unless you mkdir -p /crypto/var/empty/sshd/ , and mkdir -p /crypto/var/lock/subsys in the unmounted dm-crypt directory. It should go without saying, but if you go through all this trouble to encrypt your harddisk, you shouldn't leave backups lying around on unencrypted media.