Well let me ask you first, where did you install Grub while going thru the Suse install? It does at least tell you where its going to write it.. and what I'm thinking is its on a partition on your external drive (which might be the problem).
If your computer bios doesnt mount to your USB external drive at boot, chances are, it cant read it yet. Basically, I'm thinking your computer is booting up and looking for something not accessible.
You have 3 options (depends on what paritioning software and bios you have).
1.) If you have Partition Magic on XP, boot with your XP disc, repair your MBR and it'll boot right back to XP with all your stuff.
2.) Use the Suse install all over again and this time partition your booting drive (probably internal) to have like a 50 mb partition for your grub, and just use your external for swap and root. The setting up of the partitions to use your external may be different because of when the external drive gets mounted. Which may cause you to have to use option 3 first.
3.) Get more information about mounting to an external harddrive and when it occurs. I'm not too familiar with booting from externals, so I would do a bit of research on what your computer supports, then possibly fix your bios and boot up. If you get it right, you wont have to use option 1 or 2.
Hope this helps