Encrypted disk image plus bootable partition
 

Is it possible with SuperDuper! to backup the System folder to an unencrypted partition, and then the rest of the hard disk to an encrypted disk image?

Not in a way that would let you start up from it. You could use FileVault, though, to encrypt your Home folder and we'll maintain that encryption on the backup...

Is that because of the partioning, or some other reason?

It just occurred to me that if I partition my external HD into two, and use one partition for 'Sandbox- shared users and applications' and then the other to contain an encrypted disk image for the user files and applications, I would have what I wanted, right?

It's because the OS wouldn't know about the encryption, so how would you go about starting up?

The System folder would be on the unencrypted partition, so it should be visible as a startup volume, or have I missed something about partitioning?

There's much more to starting up than just a system folder -- you need accounts, etc.

Why do you find FileVault an unacceptable solution?

I'd like to have a minimal startup partition, and then another encrypted partition for everything else. It may seem odd to want to encrypt non-user files but I would find it useful.

Do you know of somewhere I could find out about which directories are required for startup, or is this non-trivial?

What I'd suggest is partitioning the drive. Then, install a minimal system to the first partition.

Once that's done, back up your whole system to an encrypted disk image on the 2nd partition. You'll have to create the image by hand, but we'll be happy to use it once created.