I have 2 internal hard drives, both with 2 partitions. Can I therefore use 1 large external drive to backup all of these (i.e the entire computer)? Or do I need a separate external drive to backup each partition (or drive)?
Not sure I fully understand your question, but: you just need enough room, doesn't matter if the backups go on one or many (external) drives.
How exactly are going to do the backups? Are you going to use Norton Ghost? That's a great way to go, it saves an exact copy of a partition (called an 'image') to a file. There's no more complete backup than that. Great for disaster recovery.
Well I was going to just use the integrated backup software of the (Western Digital) external drive.
Are there any disadvantages of using Norton Ghost? For example is it possible to restore a single file from the Image file that Ghost creates, or do you have to restore the entire partition? If not, then I guess I'll have to use the supplied software.
It's got a utility that lets you examine the contents of a saved image file, at least my version does (9.0), and you can pick and choose folders/files to restore. And of course you can also do the whole image as well.
I don't know of any disadvantages, except maybe that it installs as a service. Not the best for an audio machine, so what I do is disable the service until I want to use it, and I just enable/turn it on when I need it. I don't use it too often so it's no big deal.
Unfortunately I'm not familiar with Retrospect, so I can't help you there. The big difference with Ghost compared to most backup programs, though, is that it can back up a drive image, as opposed to individual files/folders. On a normal backup program, even if you were to select every file and folder on a drive and back them up, it's still not the same as saving an image - a binary snapshot of the bits on the hard drive. Doing a copy this way gets everything: registry, boot sectors, etc. There's no more complete way to back up a hard drive/partition. Even if your drive completely corrupts, if you have a recent Ghost image of it you can be completely back in business in half an hour - no re-installs, no nothin'.
If Retrospect does this then that's cool, just make sure it has this capability.