[ubuntu] Problem running Ubuntu 12.04 from external hard drive

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http://ubuntuforums.org – 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 " (Hardware)