Installing Ubuntu 11.04 or any other distribution on an external hard drive connected to a computer via USB is about the same as installing it on an internal hard disk. The key is to remember, after partitioning the disk, to install GRUB, the boot loader, in the MBR of the external disk, rather than in the MBR of the internal disk.

The resulting system can be used as a mobile backup of a traditional desktop installation. This article offers a step-by-step guide on how to get Ubuntu on a 2.5' external hard disk.