My seagate external hdd 500gb is not working and is not detected and regonised in any pc..
An external storage-device has four major components:
* the USB cable
* the power-adapter
* the disk-drive inside the enclosure
* the USB-to-disk-drive adapter inside the enclosure.
One of these components has failed.
Try a different USB cable.
Try connecting to a different USB port on your computer.
Try connecting to a USB port on a different computer.
Take a "multi-meter" and measure both the voltage and amperage output from the AC adapter, and compare with the specifications on the label on the adapter.
Open the enclosure, and remove the disk-drive, and then connect it as a "slave" disk-drive in a desktop computer, to see if it works at all.
Purchase a new, compatible, disk-drive, and install it in the enclosure, to "revive" your external storage device.
Get the part-number and serial-number from the label on your disk-drive, and access the manufacturer's web-site, and use "check warranty status", to see if they will replace the device, at minimal cost to you.