fobotcso,
I've found that SOME PCs - particularly laptops - don't have enough power for my USB external HDD. Of the 4 systems I use regularly, 1 laptop and 2 desktops can power the drive just fine through the USB port alone, but one (old) laptop cannot and requires the external power supply.
Since I also find this at work, I will generalize by saying that the older the PC the more likely it is to have the power problem.
I must also point out that I am only talking about built-in or integrated USB ports, not expansion cards of which I have no experience yet.
SD