It is very common for smaller devices not to be supplied with a power supply but to get all their power from the USB cable. It's a good idea in theory but limited by the 500mA that each USB chip can provide.

Mice and keyboards are fine, but once too many devices are added power can become a problem. Before long you can feel like Gary Sinise in Apollo 13 as he tries to work out how to turn on all the systems in the spacecraft without overloading the available power.

The problem gets worse as hubs are introduced. They draw power themselves as well as dividing the available power even further.

This problem happened to me late Saturday night as I tried to use a full size midi keyboard. It was fine for the first few notes and then wouldn't respond to pressing two keys at the same time, or waited several seconds before playing the note.

I checked the latency performance of the computer with the useful DPC Latency Checker and all was fine. I started to worry that it was the keyboard that was at fault. Then I tried a little 37 key mini keyboard. This time everything worked.

I knew the keyboard could be powered with an external PSU but it been fine on a previous computer. Just to be sure, I plugged it to a power supply and then reconnected it. All worked perfectly.

Conclusion. If you have a power supply connector on a device, use if at all possible. And the purchase of a good multi voltage adapter is money well spent. By good I mean one that has plenty of power on offer, look for around 1Amp to be on the safe side. See Maplin for an example. Most devices will have markings on the case to show how much power is required.