How to install ubuntu on an external hard drive
Step 1: Your flash drive.
One of the things you need to take into consideration is the size of ubuntu’s default install and the size of your flash media. My suggestion is something about close to 7-8gigs to be comfortable but I think a 4gig flash drive should suffice if you plan to use something like a persistant live cd, I suggest 7-8 as we are going to do a full install of Ubuntu 9.04.
Step 2: Loading your cd in live mode first
I suggest this first as it will allow you to access tools the install environment would give you. once loaded go to
System->Administration->Partition Editor
Select your drive you want to install ubuntu on (in my case it was an external Hard drive about 80gigs) and format at least 80% of your hard drive as ext4 with the mount point as / and the rest format it as a linux swap patition.
Step 3: Installing Ubuntu (finally)
now that we partitioned our flash media it’s time to install ubuntu. Double click on the install icon and follow the instructions, select your flash media to install it on and then when it comes to review the installations click on advance on the last page and select /dev/hda (all depending what ubuntu classifies your flash drive as) and this will install grub right on your flash drive and not on your computers hard drive.
Step 4: Testing it out
now once installed reboot your computer and select it to boot from a usb drive and you should see grub load 
There you have it folks Ubuntu installed on a removable storage device. Eventually I’ll add some screenshots of the process to show you guys
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I removed my HDD and inserted an external hard drive with this partitions:
1- NTFS
2- ext4 for /boot
3- ext4 for /
4- ext4 for /home
5- and a swap
after installing ubuntu 9.04, grub shows me the error 17. I searched but didn’t find the answer.
can you help me?
please email me. thank you