You're very probably right. I've had a couple of hard disk failures recently and have become a bit paranoid about having everything in duplicate. At present I use Syncback to replicate all the media folders on the main 2TB usb drive of my media PC to a second identical drive. I only have to run it once a week and it does a one-way sync, or echo, from drive 1 to drive 2. It would be very convenient to have something equally simple for the Humax and a wholly manual system is not really a convenient or foolproof answer. The ideal would be to run a script at a convenient time that would check the contents of specified folders on the internal disk against the backup, delete from the backup where the file has been deleted from the internal and put an unencrypted copy of any new files from the internal to the backup. All without running an ongoing unencrypt process on all the files on the internal disk, many of which are just temporary, time-shifted programmes.
That, after all, is what backup utilities generally do. The focus of unencrypt is, obviously, to unencrypt files - it is not primarily a backup utility though unencryption would be a unavoidable part of a backup system for the Hummy.