Install ubuntu in external hard drive using windows 7

1. Create a bootable usb[ex: pendrive] or disc[ex: cd/dvd] which contains the iso
2. Insert the external hard drive into usb slot and start windows 7 and immediately get to the boot menu by pressing F12 and select the bootable pendrive/cd/dvd
3. Follow the on screen instructions when ubuntu loads
4. Choose 'Something else' option when asked while installing
5. Carefully choose the external hard drive. The naming could be something like sdb/sdc.. etc.
6. After that under the selected drive, make the below partitions by clicking + button:
1. swap - select type as 'Swap Area' from the dropdown and size as 4000 MB(4GB)
2. home - select type as 'ext4' [probably this is the default option], mount point as '/home' and size as required [ex: 4000 MB(4GB)]
3. root - select type as 'ext4' [probably this is the default option], mount point as '/' and size as required [ex: 8000 MB(8GB)]
7. Select 'Device for boot loader installation' as the external hard drive [ex: /dev/sdb HD(16GB)]
8. Then click on 'Install Now'
9. Follow the onscreen instructions during the setup
10. Finally when it asks for 'Restart' do a restart by removing the pendrive/cd/dvd and get into the boot menu and select the external hard disk
11. It should now boot from Ubuntu