portable external hard disk

Nearly all the 2.5" drives don't need anything more than USB power.

You can make your own external drive cheaply simply by putting a disk drive in an enclosure. Some of them don't even require a screwdriver.

I strongly recommend fitting a 320 GB drive in the laptop if it only has a small one at the moment. Then an external drive might not be needed regularly at all. That would save hassle each time the laptop is used.

We had a 160GB Lacie and could not get the lap top to see it. It was exchanged for a Maxtor which worked first time. The Maxtor was a USB powered devise but it had an adaptor to take power from two USB ports. Apparently on some older laptops the power supplied by the USB ports is too low to drive some devices