Easy Question - Auto Hardware Detection

I've installed DSL to a hard disk and it works great.

However, when I cloned the hard disk and put it in a PC with different hardware configurations it seems to be pretty random at getting the video card working correctly.

The DSL live CD or clean install finds the new hardware just fine ?

Is the hardware auto-detection turned off once it's installed onto a hard disk? If so how can I turn it back on.

Thanks for any help !

New Zealand.