Installing ubuntu in mac

How to install ubuntu in mac by booting from external hard drive

1. Make a bootable pen drive for ubuntu using the iso and the dmg file.
2. Erase the external hard drive by formatting with the disk utility by changing the name, mac os x journaled, GUID partition.
3. Make the partition using the disk utility only and change its name.
4. Now restart using the bootable pen drive and also connect the external hard drive.
5. Install ubuntu and then restart your mac.
6. Changes are saved now you can disconnect the bootable pen drive.
7. Now go to the system preferences in the mac and turn off the file vault (since file vault encrypts all the data and doesn't let us access it in the forthcoming commands), then restart your pc pressing command + T and then choose terminal locate refind on the terminal and install it.
8. Your both operating systems are ready to use.

NOTE: Never use sleep mode for ubuntu and choose suspend while shutting down as it will corrupt your external hardrive and corrupt ubuntu resulting in all data loss.