How to use a `dd if=/dev/sda of=/dev/sdb` drive?

Given: a sudo dd if=/dev/sda of=/dev/sdb duplicate drive of the main boot drive.

How does one safely, reliably :

1. mount a dd duplicate drive as a second drive?
2. setup the dd duplicate drive as alternate boot drive?
3. safe practice to restore from a dd duplicate drive?

My notes so far on for Ubuntu 12.04 are as follows:

• Mount. The Ubuntu Utility Mount Volume gives the error Error mounting: mount exited with exit code 1: helper failed with: mount: according to mtab, /dev/sdb1 is already mounted on / mount failed. Is there a safe way to just mount a duplicate boot volume as a second drive?

• Boot. Function key to select the start up disk from the BIOS/GRUB appears to be the way to go. What does it mean if a blank screen is seen after the disk is selected far what seems to be a long time?

• Restore. If the duplicate is used as the /dev/sda boot drive is sudo dd if=/dev/sda of=/dev/sdb OK as a restore procedure? Or, is it safer use some third minimal boot (e.g. USB) drive for sudo dd if=/dev/duplicate of=/dev/toberestored? or something else?

Background Context: There is an Ubuntu 12.04 embedded system that I would like to migrate forward with a clean (not-upgrade) install of Ubuntu 14.04. This is a single board system with and fixed main SSD drive.

Current thinking is to create the following:

1. File Migration Disk. A mountable drive with the 12.04 files which can be seen by the 14.04 system. (alternately possible with rync)

2. Regression Test Dual-Boot Configuration Disks. A dual boot drive configuration with an external 12.04 drive or fixed internal SSD (with a clean 14.04 install) can be use to boot the system. (can dd be used here?)

3. Baseline Backup Disk. A full image, non-incremental, as-is, if-needed revert to 12.04 recovery drive. (completely separate drive from 1 & 2)

Where could and how would dd if=/dev/sda of=/dev/sdb drive process fit into the above migration scenario?