Using exernal drive with laptop

OK, guys---we've got a couple of rocket scientists (not) hard at work, and we've run up against a stump. Let me give the background of the problem, and tell what we're up to.

Got a friend who has a laptop. His hard drive crashed awhile back. He took the laptop to a pc shop and got a new hard drive installed. He was checking out options of recovering his files off the old hard drive. I told him same thing had happened to me, and the way I did it was to take out my bad hard drive, get all the numbers off it, find a new identical one, then take the controller boards off and swap the suspect bad one for the known good one, and then stuff the hard drive back in my pc. It worked fine, and I was able to copy off my files.

My friend has now done the same thing, and has his old hard drive ready to go, with a new controller board on it.

He's trying to simply get the hard drive to run as an external drive, without having to go inside the laptop and swap the new/old drives out. He's got USB ports available.

This morning, he tried using the old hard drive from a USB port. So far, his laptop doesn't let him do this. Bear in mind that the disk inside the hard drive is already partitioned and formatted, and has data on it that he doesn't want to lose.

We can eventually take the new hard drive out of the laptop, and put the old one if, if we have to. This is our approach of last resort, though.

So, does anyone have any neat tricks we can try to get the laptop to recognize its old now external hard drive?

PS: my friend does have an IDE adapter cable if that's what we should use.