random "Boot OS X" partition showing up in my boot options in my mac pro

One of the biggest criticisms of Time Machine backups to date has been, "But it is not bootable."

There has been a Utility, "Lion Recovery Disk Assistant" that would copy a stand-alone Recovery partition ONLY to an external drive, such as a USB thumb drive.

It may be that the latest software decided to do that function automatically to the Time Machine drive. Or maybe you did it yourself in a stroke of brilliance and just forgot how brilliant you really were. [I am not yet running 10.8.2, but I think I will use that Utility on my Time Machine drive -- that seems like a great place to keep an extra Recovery_HD!]

I recommend shrinking and creating a small Apple only 24GB OS X boot volume just to have it handy.

CCC will clone your system and let you skip everything non-system (keep your ~/Users/myaccount/Library to hve it handy) and that Recovery partition.

Disk Warrior is known to case an "EFI Boot" in boot manager (Option key choices)

Some prefer to clone the system and use TimeMachine and not have the system included as part of TimeMachine.

And then there is Lion/ML Disk Maker to create a bootable installer - so you can do a clean install later and not have to redownload.

I have noticed the exact same thing, since I did the 10.8.2 update. I now see a Boot OS X drive when I hold the option key when booting, and I have a Boot OS X volume on my Time Machine drive. It has 134.2MB allocated to it on the TM drive, so I assume it contains the operating system files in the event that the internal HDD fails. If the internal HDD is OK, it just boots from that one, even if you select the Boot OS X.

I struggle with the fact that Apple makes these changes, but they are not documented or communicated (or at least I cannot find any documentation or communication). I have spent a LOT of time trying to figure this out, and still don't know for sure what has happened, or why.

Sounds like the hudden small EFI partition, which is probably the bootstrap anyway.

I have the same volume showing up. It is definitely associated with my Time Machine disk. If I plug in my Time Machine drive (which was formatted with 1 partition), running df -h at the terminal shows me all of my normal volumes as well as the Time Machine volume. As soon as a backup is run, I see the extra "Boot OS X" volume. The volume doesn't show up in Finder or Disk utility.

Additionally, there is an annoying problem associated with this. Ejecting the time machine volume via terminal or Disk Utility results in a "Disk was not ejected properly" message on disconnecting the drive -- presumably due to this extra hidden volume. Ejecting my time machine volume via finder results in a message stating that there are two partitions on the disk, would I like to eject them all. Choosing eject all means I can disconnect the disk without any warnings about it not being ejected properly.

But again, the Boot OS X thing only shows up after running a TM backup. Weird.

I have a bug report open with Apple for this one.

Came across via google and I have the same effect. It is Time Machine. It's running on an external hdd connected via firewire. This hdd contains two partitions, one für data, one for Time Machine. So I connect the external hdd to my MacBook Pro and if I want to remove it without having done a backup, it says "...contains TWO partitions, are your sure....". If I want to remove the hdd having done a backup it says "...contains THREE partitions, are you sure..." But in disk utility only two partitions are shown. I was a little bit confused, because this features seems not to be documented by Apple.

I'm using a Mac Pro 2 x 3 GHz Dual-Core Intel Xeon running 10.7.5 Lion. I also show this in Disk Utility while using the Debug menu. My "Time Machine" is in Bay 2 of 4 and Bay 2 is where the "Boot OS X" shows up. I don't use Boot Camp and I don't have multiple operating systems (Multi Boot).

I think Time Machine may have done an ordinary back up of your EFI (boot) partition, normally invisible, but about 210 MB.

That would explain what it is, how it got there, and why when you choose it to boot from, it boots to your main drive.

Recovery HD is about 650MB.

Using Disk Utilitiy would cause "EFI" on Option boot.

The EFI partition were showing up back in 10.4.x and confused me at first as TechTool Pro 4.x was showing them and Micromat did not know.

Using Disk Warrior will also cause the EFI partition to show on startup .

It is usually seen as FAT and should also not and unlikely could/would - I'd send an email to Bombich or ShirtPocket and ask about it if it is a problem and not just cosmetic.

This causes a problem to launchd to. If, as I have triggered a synchro prozess with rsync with condition StartonMount. It will be triggered every hour together with TimeMachine. Thats grnff..