Windows 7 - External Hard Drives without their own Power Supply are not detected.

First of all I am sorry of this has been answered somewhere, but I haven't been able to find it and I really need a solution.

My desktop does not detect and external hard drive that do not come with their own power supply. I have many externals that also plug into the wall and they are working fine; but whenever it is an external without a plug, i.e. the portable ones they are never found.

They do not show up in Computer Management, so it is not because I did not assign a letter to the drive.

I have tried various things, but I don't know what I should be testing for or looking for.
These externals all work on other laptops and other desktops perfectly fine so it is not the externals. When they are plugged into my desktop, they light up normally like they should, but the desktop never detects them. I have tried these externals with the different USB cable I have on hand, but nothing seems to work. It doesn't matter which USB port I plug the device in, it still gives me no response.

I am running a i7 860 on a ASUS P7P55D Pro.
If anyone has any information or a ideas please let me know.

Thanks.

This is a rather common, but sporadic, problem. The issue, I believe, is that the USB ports on the problem PC are not putting out the specified power to run the drive. This is a problem typically seen in laptops.

IIRC, USB 2.0 specifications require the supply of 2.5 watts of power over a maximum of 5 meters at 5 volts (0.5 Amps). If you are ambitious you could test for the correct specs with a good multimeter.

The other possibility is that the problem external drive is requiring more power than is supplied by the USB specifications. This is not likely in your case because the drive works on other PCs.

One work-around, in either case, would be to used a powered USB port for that PC.

I agree with TVeblen.

This should normally not happen on a desktop. Either the motherboard is deficient in not supplying enough power to the USB ports or the power supply 5v rail is deficient. One has to replace them one by one and check.

If your system is a branded one and still under warranty, you should throw the problem to the OEM and get it resolved.

If it is a custom-built one check to see what the builder can do.

Another workaround but not a solution: You may try to use a Y cable for the external, plugging it into two ports on your PC. This still may not be as reliable as running them on an externally powered USB hub.

IMO, you should not go with any of these workarounds but get the problem resolved.

I have this board and can verify that it's USB ports should be putting out enough power for external drives. All I can suggest is you make sure the Intel chipset drivers are up-to-date