I have 20 GB HDD which can'not detected is any software detect it

If the drive can be seen by device manager but not under My Computer, right click my computer and go to "Manage". (Check out: right clicking my computer icon on your desktop, select disk manager)
You should see your hard drive either lister as foreign or not initialized. If foreign, it will give you an option to import and is not initialized, it will allow you to initialize it.
This will hand over drive management to the OS and you should be able to see it in My Computer.
Have you ever worried about hard drive data recovery? Here is the solution:
You can recover data from hard drive with data recovery software like Tenorshare Data Recovery. It can recover data after deeply scanning your hard drive to look for lost data. You can get it from: http://www.any-data-recovery.com/index.htm

try other usb port, or other computer and see whether the hdd is detected. If still not, try to go to device manager ( http://www.computerhope.com/issues/ch000833.htm ), and then assign a drive letter to the hdd. After that, try to access it from Computer, if not able to access it, format it (use quick format) and then use this data recovery program to recover lost files.
http://www.facebook.com/pages/Asoftech-Data-Recovery/290956907592436?sk=info

try other usb port, or other computer and see whether the hdd is detected. If still not, try to go to device manager ( http://www.computerhope.com/issues/ch000833.htm ), and then assign a drive letter to the hdd. After that, try to access it from Computer, if not able to access it, format it (use quick format) and then use asoftech data recovery software to recover files.
http://www.asoftech.com/adr/

try other usb port, or other computer and see whether the hdd is detected. If still not, try to go to device manager ( http://www.computerhope.com/issues/ch000833.htm ), and then assign a drive letter to the hdd. After that, try to access it from Computer, if not able to access it, format it (use quick format) and then use this program to recover your files.
http://www.asoftech.com/adr/

try other usb port, or other computer and see whether the hdd is detected. If still not, try to go to device manager ( http://www.computerhope.com/issues/ch000833.htm ), and then assign a drive letter to the hdd. After that, try to access it from Computer, if not able to access it, format it (use quick format) and then use this file recovery program called asoftech data recovery to recover your files.
http://www.asoftech.com/adr/

In this case you should use data recovery software to try recovering the files and data, provided those lost data are not fully overwrtten.

Be careful: Before your files are recovered, do not attempt to save more files to the HDD in case the original files are overwritten.

My External HDD of Western Digital (500 GB) not detecting by Win-XP SP-2. But at the same machine another External HDD of Western Digital (160 GB) is detecting and functioning well.
What is the problem? & what to do to get back my necessary files stored in the 500 GB HDD.
Can any one help me, please? URGENT.My External HDD of Western Digital (500 GB) not detecting by Win-XP SP-2. But at the same machine another External HDD of Western Digital (160 GB) is detecting and functioning well.
What is the problem? & what to do to get back my necessary files stored in the 500 GB HDD.
Can any one help me, please? URGENT.

If the drive is corrupted the data is useless.

is the jumpers set correctly . to slave or try cable select

Does your hard drive get detected in the BIOS? If yes then you can recover data by plugging that hard drive as secondary and running a recovery application like Recover My Files.