If your USB drive is not being recognized by your computer when you plug it in
try unplugging it, and also unplugging any other USB devices that are connected.
All USB devices, even those that have an external power supply, draw a portion of their power directly from the USB bus.
If you have too many devices connected to the same USB controller, problems such as a drive not being recognized can result.
Incompatible File System
The Mac-operating system can read drives formatted under the FAT32 and NTFS file systems used by Windows, but Windows will not recognize HFS, the file system used by Mac OS.
If you connect an HFS-formatted external drive to a computer running Windows, the drive will spin up but Windows will not respond.
The drive will have to be used with a Mac, or reformatted using a disk-partitioning utility.
Drive Disconnected from Power
Some external hard drives have the option of drawing all their power from the USB bus, or receiving supplemental power from a wall outlet.
If your drive will not operate using power from the USB bus only, connect the power supply. Many drives do not have this option; they will not operate unless they are connected to both a power outlet and a USB port.
Driver Update Required
Some older motherboards may require updated USB drivers to properly utilize a modern USB 2.0 device.
If you have exhausted all the basic troubleshooting steps above, visit the website of your motherboard's manufacturer and look for updated USB drivers.
Also, ensure that your Windows installation is fully up to date.
Corrupted Files
USB hard drives are susceptible to data corruption if either the power or USB cables are disconnected during a write operation.
Over time, this could lead to a situation in which Windows will be unable to write to the drive correctly.
To have Windows attempt to fix corrupted files, run Scandisk by right-clicking on the drive after double-clicking on "My Computer." Click Properties > Tools > Error Checking. Click the two check boxes to have Windows attempt to fix errors and bad sectors.
Click "OK" to start the operation.
Depending on the size of your hard drive and the speed of your computer, this operation can take a few hours to complete.
Failing Hardware
If your drive is still not being recognized, but it is making audible noises when it is plugged in, the drive may be failing.
Nearly every hard drive manufacturer distributes a free utility to check the drives that they make for errors.
Visit the manufacturer's website to download this utility.
If the drive is failing, the utility will most likely be able to detect it.
Contact the manufacturer for a replacement if the drive is under warranty.
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