I finally decided to install Ubuntu 12.04 LTS on my laptop; I also upgraded the internal disk to 1TB and kept (untouched) the old 500 GB disk in a SATA-USB enclosure for sake of security (you may never know...).
Now, I need to boot again the old disk from the USB. When I choose to boot from the USB disk, Ubuntu 10.04 splash screen is displayed; after few seconds I get the following error message:
ALERT! /dev/disk/by-uuid/affc8cb...... does not exist. Dropping to a shell! BusyBox v.1.13.3 (Ubuntu 1:1.13.3-1ubuntu11) built-in shell (ash) Enter 'help' for a list of built-it commands. (initramfs)
I'm not very skilled with this stuff. How do I get Ubuntu 10.04 to start from the USB disk?