Hello Charlz
1. This is the reason why I said - get a bigger Internal HD. You will run out of space very soon. You then have to transfer some of your pictures to the external drive - and delete them from the internal HD because you need space there. The moment you do that yo do not have a backup anymore because the archive pictures just reside on the external drive only
2. Regarding GUID, just click on Disk Utility, select a drive (not a volume), click on Partition and the on Options. It will give a short overview when you need GUID.
3. A lot of people define backup as having an identical file 3 times and preferably on different storage media. One of those storage media should be offsite in case of desaster. You can only answer for yourself how important/replacable these files are for you.
4. I had in the past a lot of external drives flying around and it gets a nightmare to manage. I just bought a Drobo from Data Robotics. This box is really great. Have a look at their website. I do not work for them or anything. I just read an article from Scott Kelby on it and had a go.
http://www.scottkelby.com/blog/2007/archives/735
Sigi