Hi,
1. Did you make any changes to the computer before the issue occurred?
2. Do you get any error message while you connect the external hard disk?
I would suggest you to try the steps mentioned below and check if it helps.
**Method 1:**Try connecting the external hard drive in different USB port or try connecting the hard drive in different computer and check the issue persists.
**Method 2:**Try to run the Fixit tool from the link below.
Hardware devices are not working or are not detected in Windows: ****http://support.microsoft.com/gp/hardware_device_problems
**Method 3:**If the above step fails then check if the hard drive is detected in the disk management.
To open Disk Management
a) From the Start menu, click Run.
b) In the Open box, type diskmgmt.msc, and then click OK.
Method 4: Also try to follow the steps from the mentioned link below.
A computer that is running Windows XP cannot detect a USB flash drive, an Apple iPod, or an external hard disk drive: http://support.microsoft.com/kb/925196
Hope this information is helpful.