I am using Intel D945 GCNL board. When using 3G USB data card in one of the USB port, other USB ports becomes nearly functionless i.e sometimes working and sometimes not. specially the front USB becomes workless.They do not read any pendrive or other equipments. And when 3G data card is plug out, the ports works normal. I have checked the voltages in the USB ports in both conditions i.e. when the 3G card plug in and plug out. In both cases there is no major difference. Only a voltage drop of 0.01 V is observed when 3G data card in plug in. If there is a solution please send the solution to akd260671@gmail.
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This sounds like one of 2 things. First, your power supply may have an issue powering everything in your system and the 3G card is putting it at its threshold. I have seen a similar problem with small PSUs in HTPCs and too many USB devices were attached to them.
The other issue could be the front panel connectors. Those are often of poor quality and simply can't handle the electrical load required by a device such as the 3G card.
Have you tried connecting everything to the back panel connectors and nothing in the front panel connectors? Also, what make/model of power supply are you using? Last, what are the specs for the rest of your system?