Ok ... I'm assuming you are going for the 3.5" External HDD. Also In terms of price, seems like you will be going for the best i suppose.
Here are the options for you to buy:
* Seagate FreeAgent™ Pro External Hard Drive 500GB - USB 2.0, eSATA, FireWire
Dunno abt its price in India ... but its the best. (has all kinds of connectivity options)
* Seagate FreeAgent™ Desktop External Hard Drive 500GB - USB 2.0 ........... Rs. 8200
* Seagate eSATA 500-GB External Hard Drive ......... Rs. 7800
Others:
* 500 GB Maxtor Ext 3.5" + IEEE ........... Rs. 7780
* 500 GB W D USB + IEEE .......... Rs. 7700
* 500 GB W D USB .......... Rs. 6500
* 500 GB Iomega Ext 3.5" IEEE........ Rs. 10200
* 500 GB Iomega Ext 3.5" .......... Rs. 8300
I personally recommend you to go for the Seagate FreeAgent™ Pro External Hard Drive if available.
else the Seagate FreeAgent™ Desktop External Hard Drive.