USB Device is not detecting in DEll vostro A 840

I devised these simple steps that tend to remedy quite a few issues with most USB portable / external hard drives and OTHER devices (though not always)

A few things to check but assumes USB and Windows for other interfaces / operating systems (mac/ Linux) similar steps may be adapted to suit.

1. Ensure it is connected directly to the computer to a USB 2.0 port not a USB 1.0 port as this can have effects on performance and reliability

2. Use only the cables that came with it NOT one that fits that may have been lying around or is longer. Not all USB cables are equal even though they should be)

3. Do not connect through an external USB HUB unless that hub is USB 2.0 AND has its own power supply.

4. Use ONLY the power supply that came with it if it has an external power supply Don’t use any other unless you know it has both the same voltage and current rating e.g. 12V 500mA anything rated below that would not work properly.

5. Always use the same port for connecting your devices. Some devices do not like being switched about. If switched they may want to install software / drivers again. This can be especially true if you move a HUB to another port

If you checked and fixed anything there and still have issues then check your hardware from CONTROL PANEL / SYSTEM / HARDWARE.
Any exclamation marks by hardware need fixing before you investigate any further

USB flash drives can fail for no apparent reason.
Sandisk Cruzers are especially bad for this.

USB hard drives should be treated with extreme care.
This is a real hard drive ! and knocks and bumps can quite literally kill them.

It is possible to remove drives from the enclosures in most cases and connect directly inside a PC to fully check it.

Note that if you hear a drive clicking or pinging this could be the worst thing you would hear.
Known as "The Click Of Death"

If none of the above steps help look at updating the drivers from the manufacturers web site in the support section.

HI
I HOPE THE FOLLOWING STEPS WILL HELP YOU TO SLOVE THE USB PROBLEM

1. Step 1 Plug the drive into a USB port that you know is working on a separate computer to make sure that the port itself is not causing the problem. Place the drive back into the original USB port if it remains undetectable on another system.
   
2. Step 2 Mapping a Network Drive Open the "Start" menu and click on "My Computer" to bring up a new window that lists all of the drives connected to your computer. Take note of the letters assigned to the drives connected to your computer. Click on the blue button at the top left end of the window labeled "Map a Network Drive." Click "Continue" if a second window pops up asking for your permission to open the new menu.
   
3. Step 3 Network Drive Mapping Menu Click the "Drive" button at the top of the new window. Select a drive letter from the drop-down menu that is not already in use, such as "Z" or "M." Click on the "Browse" button and locate the USB flash drive that your computer hasn't been detecting. Click on the drive and then click the "OK" button.
   
4. Step 4 Control Panel Click the "Finish" button to close the window and map the USB flash drive to your computer. Restart the computer and then return to the "My Computer" menu and check to see if the USB flash device is usable. Navigate to the "Start" menu if the device still isn't working properly, and open "Control Panel."
   
5. Step 5 USB Device is not detecting in DEll vostro A 840 - 206581_thumb.jpg Administrative Tools Click the "Administrative Tools" link in the Control Panel menu. Click "Continue." Double-click the "Computer Management" link at the top of the menu. Click the "Disk Management" button at the left side of the window to bring up a list of drives on the computer. Right-click the USB flash drive and choose the "Format" option. Click "Start" and wait for the formatting process to finish. Restart the computer and then attempt to use the USB flash drive again.