[ubuntu] ISO for CD to boot USB hard drive

I have recently installed Ubuntu 15.10 on a 480GB SSD on my Packard-Bell Easynote laptop. I have re-installed my old /home via backup and all my old packages via Synaptic so I have a pretty good replication of my old system. My old HDD was 1000GB with three main partitions, a small Windows partition (because it was paid for with the laptop!), an Ubuntu 64-bit 15.04 partition (380GB) and a Ubuntu 14.10 32-bit partition (480GB).

Now that the Ubuntu 15.10 on the SSD is stable and working well I do not want to keep removing it and plugging in the old HDD to check things on my old working systems, so I have coected my old HDD to a SATA/USB cable and can read all of the partitions by plugging it in to a USB port. I would also like to be able to boot into the old system via USB so I altered the BIOS so that USB-HDD came above the SSD in the boot sequence. However, on boot, the HDD is ignored and the system boots straight into the Ubuntu 15.10 on the SSD.

I am not sure why the HDD doesn't boot, maybe the partitions are simply too big, but I thought that maybe the solution would be to write Grub script on a bootable CD that would allow any of the partions on the USB-HDD to be booted. Is this possible and if so how?