I spent a good deal of the afternoon diving in where no sane person really wants to spend a lot of time -- in my Lion recovery partition. It's not hard to get there, and it's quite a curious place when you do. [For those commenters wondering about the use of photos rather than screenshots to illustrate this post, it's hard to take screenshots on a system where the boot volume is read-only. –Ed.]
The Recovery volume is a small slice of your hard drive that gets partitioned off during your Lion install; it's not optional, because that's actually where the OS gets installed from. You can view the contents of the Recovery volume by mounting it with the command-line diskutil tool, as John Siracusa points out; the regular Disk Utility app is thoughtful enough to keep it hidden.
To restart in recovery, reboot your computer and hold down Command-R after the chime (you can also use the traditional Option-key holddown, which will show all your bootable volumes including Recovery). Before long, the gray linen background appears and the Mac OS X Utilities window pops up.
The OS X Recovery partition includes a number of built-in utilities to handle system recovery tasks. The Utilities window allows you to
- Restore from a Time Machine Backup
- Reinstall OS X
- Use Disk Utility to repair or erase your hard drives
- Browse with Safari to get online help.
Choosing the Safari option opens a web browser that immediately takes you to a basic help page. This help page is stored locally on your recovery partition at the following link:
You are not limited to Apple pages, however. The reason I was able to get that link up there isn't because I wrote it down. I copied it to memory, and pointed Safari to Earthlink's web mail page and simply e-mailed it to myself. I had no problems accessing any of the (admittedly limited) pages I tested.
From there, I explored the Mac OS X Utilities > Utilities menu. Located off the main help screen and in a windows sub-menu, you can manage your firmware password, test and fix connectivity issues or access the Terminal for command-line management.
Of course, I had see what Terminal had to offer. Turns out that your entire file system mounts, if it can. You can navigate to your user folders and access any material located there. I did not try it out myself, but I imagine you could attach a USB drive of some kind and copy files over if you needed to. There is no authentication here, so it's also possibly a bit of a security hole for anyone with physical access to the system. (For those who are concerned about physical access, don't forget about FileVault and/or an Open Firmware password to keep things secure.)
The Recovery boot volume is read-only, and has a very limited set of files and features. You're actually running from the system image stored in BaseSystem.dmg, which gets mounted by the startup executables inside the com.apple.recovery.boot directory.
That doesn't mean you can't run Nethack from your recovery partition. Just make sure your install is set up to run completely on another drive -- which mine is. (Also, don't forget to re-compile it from scratch. The PPC version no longer works on Lion.)
Once you're done exploring, boot your way back to your primary partition and let your Recovery partition rest -- hopefully for a long, long, long time.
Apple OS X Yosemite