Is is possible to install BackTrack5 and Ubuntu 11.04 on a single external hard drive(seagate)?
If it is, how can I do it?
Are you asking if you can have both Operating Systems installed on one hard disk? The answer is yes, if you have multiple partitions on the disk. If you have two partitions, one for Ubuntu and the other for BT, they'll operate independent of each other. Either one gives the option to install the GRUB bootloader which will allow you to choose which OS to load when you turn the machine on.
I'm not sure that I understand exactly what you are asking. If you want to install BT5 on Ubuntu then you could install it in one of the myriad of virtual machine options you're given.
Otherwise you could try adding the repos
> Type: Binary URI: http://source.repository.backtrack-linux.org
> Distribution: revolution Components: main microverse non-free testing
>
> Type: Binary URI: http://32.repository.backtrack-linux.org
> Distribution: revolution Components: main microverse non-free testing
>
> Type: Binary URI: http://all.repository.backtrack-linux.org
> Distribution: revolution Components: main microverse non-free testing
Your best bet would be to go to the backtrack website and check the forums for compatibility.