I have a usb hard drive (60gb western digital 2.5", Rosewill case) that I use to back up my laptop (Dell Inspiron 1505). Attaching this usb hard drive to the Dell machine has never been a problem. It has been recognized immediately. I attempt to attach the usb hard drive to a second computer (Compaq/HP nc6400) and it isn't recognized. Occasionally on the HP machine I will get the pop-up that usb hardware has failed to install. Typically, I get no message. I cannot see the drive in Control/Admin/Computer Management/Drive Management.
Both machines are running WinXP Pro SP2. I have full admin rights on both machines. I typically use a usb cord that plugs into 2 usb ports on the laptops - allowing the drive to draw power from multiple ports as needed although I've run it with the power from a single port as well. One more bit of information about the HP machine - it does recognize a Sandisk USB flash drive with no difficulty.