[ip6ix]

I'd be tempted to use a pipe, and use tar or pax with openssl for the encryption, and write directly to the disk. For example, assuming your USB disk is sd1, to backup the /home partition something like the following should work:

tar cf - /home | openssl enc -des3 -salt | dd bs=10240 of=/dev/rsd1c

You'll be prompted for a password. To restore, do the reverse:

dd bs=10240 if=/dev/rsd1c | openssl enc -d -des3 -salt | tar xpfv -

For infrequent backups the above may be enough.

Have you taken a look at sysutils/duplicity?