Over the last few years, devices using USB ports have swept virtually all other connection types from store shelves - and that's a good thing. You no longer need to worry about choosing between serial or PS/2 mice, parallel port or SCSI scanners, parallel or serial printers - USB rules! However, there are still a few potential gotchas to consider when wading through holiday shopping crowds on a mission to pick up a USB device for your favorite computer user. These include:

• - Bus-powered device blues
• - Port-grabbing USB hard disks
• - USB versus Hi-Speed USB hubs and devices

Beating the Bus-powered Device Blues

There are three places you can plug in a USB device:

• - a USB port built into the system
• - a USB port in a self-powered hub
• - a USB port in a bus-powered hub

If you want to avoid problems, the first option's the best. With rare exceptions, USB ports built into your system provide a full 500 milliamps (mA) of power per port, the maximum power amount required for many of today's most popular USB devices like portable hard disks and flash memory. But, if you're short of USB ports, it's time for a hub.

Self-Powered Hubs Coming to an AC Outlet Near You

USB ports in a self-powered hub also provide a full 500 milliamps (mA) of power per port. By the way, the term "self-powered" is a bit of a misnomer: there are no atomic batteries or turbines inside a self-powered hub. Instead, a self-powered hub uses an AC adapter "briquette" that plugs into a wall outlet or surge suppressor. Most include four USB ports, but some include as many as seven.

'Bus-Powered' Means Low-Powered

A bus-powered hub might seem easier to use, because it doesn't use an AC adapter. Unfortunately, because it is powered by the upstream USB port, a bus-powered hub provides only one-fifth the power of a self-powered hub: just 100 milliamps (100mA) per port.

100mA is enough power to run most mice and keyboards, as well and devices with their own AC power source, such as 3.5-inch external hard disk drives, printers, and scanners. However, guess what USB devices are most likely to be stocking stuffers this year? USB flash drives and 2.5-inch hard disks! They need 500mA of power to run. A well-behaved device will complain (via the notification area) if there's not enough power, but I have seen a USB flash drive contents destroyed by plugging it into a bus-powered USB hub. Ouch!

Power Check, Power Check, 1-2-3-4...

By the way, if you're wondering how much power your USB devices use in Windows, open Device Manager and expand the Universal Serial Bus controllers category. Open the properties sheet for each USB Root Hub and Generic USB Hub, click the Power tab, and you will see the power usage for each device and the amount of power available per port. Some devices don't draw much power, and are happy with any type of a USB connection:

However, portable USB drives and flash memory devices top the list of power-hungry devices that expect a full 500mA of power on the USB banquet table:

To learn how to cope with reduced root hub power on some systems, read on.