Is this an old drive that you bought an empty USB caddy to hold?
Does the caddy have a separate power supply? Or does it rely purely on USB power?
It is the single biggest problem I have had with USB hard disks. I have one 2.5" caddy that works fine on some laptops, but that needs an extra power supply (cable from the mouse port) on others. Symptoms were similar to what you describe.
I have also had a similar problem using a USB disk attached to a Netgear wifi router. It was "supposed" to make the disk available to share both locally and over the internet, but it never worked properly. The disk is fine (LaCie 1/2 terabyte - sounds better than 500GB!!), but the router was crap. Replaced with a Linksys, and the disk just attached permanently to my PC.
SD