hi I assume that you are external hdd is connected through USB interface. The catch is your computer BIOS should support USB BOOT feature. If this is present you should be able to do it.
I have a laptop that recently the hard-drive has stopped working and so effectly the computer dosen't work at all. I bought a Freecom 30GB Mobile Hard-drive and attempted to install Windows XP Home to the HDD. The HDD was displayed and i pressed [Enter] to start installation into that empty Partition. I then got an Error message saying that my computers start up program could not gain access to the drive and could not complete. On other forums i discovered that other people have installed the contents of the Windows XP Disc to the HDD and booted from a Win98 boot floppy and then ran i386/winnt. Problem is, i don't have a floppy drive. I need to be able to boot from the External Hard-drive, any ideas ? Any help will be much appreiciated