Well, without any information what is happening when you try to boot from the external disk, it is hard to discuss any possible reasons…
May be instead of installing bootloader on the external disk MBR you installed it on the boot sector of the partition? Or may be Dell wants to boot from an “active partion” and you didn’t set the SuSe partition as active (may be this is a longshot, but sometimes Dell is quite particular…)? If you can boot the laptop from some linux “live cd” and inspect your usb-hd with fdisk, you can get some useful information for fixing the problem.
First, I would not really advise you to boot your Linux or any type of operating system from your external hard disk drive. This is because it might be the reason for hard disk drive problems in the future. External or portable hard disk drives are manufactured and designed just to store your files as backup files.