I have two desktop computers, a Dell running ubuntu 10.04, and an Acer running win 7. I wanted to try out ubuntu 12.04 without messing up anything on the two computers, so to be safe, I decided to install it on an external hard drive.
I plugged a Western Digital My Passport Essential 500 GB USB 3.0/2.0 Portable External Hard Drive into the usb port on the Dell, and to be extra safe I disconnected the internal hard drive. I put the ubuntu 12.04 CD in the CD drive, turned on the computer and did a fresh install to the external drive. It worked OK, but when I rebooted I got a message "error:invalid extent. grub rescue". I repeated the whole process a second time and got the same results.
Just out of curiosity I plugged the external hard drive into the the Dell, changed the boot order settings, and to my suprise ubuntu 12.04 booted up just fine.
What do you think is going on? Why won't 12.04 on the external drive run on the Acer (where I did to original install), but runs just fine on the Dell?
One other thing. I reconnected the win 7 hard drive in the Acer, and using windows explorer I was not able to see the external drive.