What options are available for setting up automated HD backup?

I have a second, 7GB hard drive, and would like to have the contents of my /home directory (no more than 2 GB) backed up periodically in the following fashion:

Two partitions will be made on the second HD.
The first time a copy of /home is made, it is sent to partition 1.
The second time a copy is made, it is sent to partition 2
The third time, it overwrites partition 1.
The fourth time, it overwrites partition 2.
etc.

There are two issues here:

1. The computer is not on 24/7 and it is not on every day, so I can't select specific times or dates to perform this task.

2. I also need some way to access this data. Assume that my first hard drive, containing Linux, is rendered inoperable, how could I access the data on the second hard drive?

The second half of your question is easier than the first, all you'll need is a linux rescue disk, once you boot off of that you can mount any drive you need to... Most Distribution disks have a "linux rescue" option at boot, so instead of just hitting enter to go into installation mode, type "linux rescue" and hit enter, it will provide you with some tools to fix, and recover data.

To do the first portion of your question check out unison:

http://www.cis.upenn.edu/~bcpierce/unison/

It's a fantastic file synchronizer. I user it personally to synchronize my files to a windows file share which is in our data center and backed up nightly. So, combine unison with a cron job and a bash script that runs every hour or so, but only backs up once a day, and you're set... It's a fairly straight forward program. The other option of course is a simple cron tar job which tars and zips the specified files at a specified time...

HTH,
Mike.

A friend of mine said there is a way to make two partitions, of equal sizes, and on different HDs, setup such that when a file is saved it a copy is saved to both partitions, thus making an instant backup. Does anyone know what this is called/where I can get instructions for this setup?

Sounds like the Unionfs.
http://www.linuxjournal.com/article/7714