installed Ubuntu 15.0.4 on my external hard drive using this tutorial "http://linuxbsdos.com/2013/10/23/how-to-install-ubuntu-13-10-on-an-external-hard-drive/". It is working all fine but i want to run it on my other PC as well from the hard disk. Whenever i try to boot Ubuntu from the hard drive on my other PC, it says boot manager is missing even though i installed grub on my external drive. Is there any way to do this?

Shouldn't you just install Ubuntu onto the hard drive on that computer? What am I missing here?

That is an obvious answer my friend but my question is can i make ubuntu portable by running it on multiple computers using just an externel hard drive.

Two ways I can think of, off the bat:

1. Live media such as CD/DVD or USB and then save any data to a thumb stick, or secondary partition on the boot stick.
2. Have a read/writeable drive where you fully install Ubuntu. Same result except you do not have to have secondary media to save settings and such, plus if you customize things like bookmarks or add-ons to a browser or install other programs, they will be there as opposed to a live media boot where it would always be the same.

I would disconnect any other drives and just have the install media and the destination external hard drive on the system to perform the install.

If the computer that the externel distro was created on is a bios computer and if the other computer boots with uefi may be the problem.

The Device for bootloader installation in your tutorial shows that you should install Grub to /dev/sdb. That would be correct and should install Grub code to the MBR of that drive. That will only be correct if you have only one other hard drive and the device you wanted to install to was actually /dev/sdb. One other thing that could happen is if you selected /dev/sdb to install the bootloader and went back and changed something in the partitioning, it could have reverted back to /dev/sda.

Mixing MBR and UEFI will create problems in almost all cases, not sure what you have. If you haven't resolved this, it might help if you booted the Ubuntu install medium and went to the boot repair site below, download, install and run it by selecting the option to Create Bootinfo Summary. You can either review it yourself to see if you can resolve it or post it here for help.

Yes, we need to know more about these computer.

If bios then on second system go directly to bios. Does this usb show up as a hard drive choice and is it the first in that order?