I would like the data partition separate from the os partition so if the os partition is trashed or just re-installed the data is not lost.
You could do worse than taking jimwilk's advice. But I think you should be aware of a few details related to how files are organized on Linux.
1. Most partitions have to be mounted before you can access them. They are usually mounted somewhere under the / partition (which means on a mountpoint directly under / or under a subdirectory of /, or under a subdirectory of a subdirectory of /, and so on). There are exceptions: the swap partition is mounted but not under /, and you can watch a DVD without mounting it.
2. The place where a partition is mounted is called a "mountpoint".
3. A mountpoint is simply an empty directory (a.k.a. a folder), and if you are going to specify how a partition is to be mounted in /etc/fstab, you can create the mountpoint almost
wherever you want. (I would leave the /proc and /dev directories alone though.)
4. If you let HAL do the mounting, you won't have to create a mountpoint, because HAL will create it on the fly under /media. You can't let HAL mount / or /home though. (But you may be able to let HAL mount a separate data partition.)
5. A user's home directory (/home/<username>, sometimes abbreviated as "~") is intended for that user's personal setup files and
6. It is a good idea to give its parent directory, /home, a partition of its own, because then you can reinstall your system without overwriting either
your users' configuration files or
their data -- if you just don't format the /home partition. Normally you'd then want the /home partition to be quite large.
(7. The systemwide configuration files go under /etc, and usually you can't retain them when you reinstall.)
8. But you may want to store your data on on still another partition instead of intermixed with your setup files. This is particularly useful if several users need access to the same files, or if you need to format the data partition with a Windows filesystem because you want to access them from both Linux and Windows.
9. If you have your userdata on a separate partition, that should be the large one and /home can be quite a bit smaller.
10. If you only have one user (apart from root) you might also consider mounting the separate data partition somewhere under
/home/<yourname>. That would be slightly more standards compliant than creating a mountpoint directly under /.