Yeah, I googled this, but I still don't understand exactly what information is stored on it, in that 1.39 GB. The disk utility does not allow it to be erased.
I realize that simply erasing the hard drive should be fine. I am just wondering if "Mac OS X Base System" is a partition of that drive that just can't be accessed (or erased as I am trying to do.)
It's exactly what it says. It includes the recovery partition which consists of a base OS X system to boot from and a bunch of utilities. When you hold Option at bootup it boots from that system for installing and fixing problems.
I think if you reformat while booted onto it, it won't delete it. Not sure how it works, just that it works.
However if you format from somewhere else, you might delete it. For instance, when I had both Mountain Lion and Mavericks installed onto two partitions, my disk had two different recovery partitions. One for ML, one for Mav. When I decided to replace ML and get rid of Mav, I simply reformatted the Mav partition from ML and it got rid of it.
So your answer is thusly: If you restore from the recovery, it won't delete it. But if you reformat from outside, it will. I don't think it's a partition at all. Just a special way of booting. It's most likely a disk image in a secret place on the OS X partition that is mounted when you recover. I think if you happen to reformat the partition itself from the recovery installer, it will store the disk image somewhere to reformat and then put it back afterwards. Either way it seems smart enough to remove any recovery partitions when the associated version of OS X is also erased which makes me think it's a DMG somewhere.
Observations:
When I tried to add a new partition to my disk a few years ago while I already had a second partition on it, Windows would not let me install into BootCamp because it said there were too many partitions. Which, if the recovery is a partition, then would make 3 which is too many, but if it's not a partition, it'd be 2. Maybe they fixed it recently. Maybe it used to be a physical partition, but isn't now. (I'd have to do more tests again. But I'll do that when I get a bigger internal drive.)
I have HardwareGrowler installed and I notice at certain times, mainly at software update time, this partition gets mounted for a few seconds and unmounted afterwards. So it's definitely not a partition that is around at all times. It's probably a disk image.
Short answer: It's HFS magic and we shouldn't think about how it works. Just that it works.