Question: I have been given a Mac to repair after a failed update, and may not be able to get reliable information about which update was being applied, or, indeed, to which operating system. If the system would boot, it'd be obvious which OS it was running, and could easily be confirmed by running sw\_vers or going to "About This Mac".

How can I determine which version of OS X this box has on it, by looking at the filesystem in target disk mode?

Answer: 
`man sw_vers | tail` shows you the file you need to look at, namely:

```
/System/Library/CoreServices/SystemVersion.plist
/System/Library/CoreServices/ServerVersion.plist

```

When you `cat` those files, you learn what you need to know.
I'm hoping it boots with the hardware but that the software is the only thing that's fubared...in that case you should find some hints if you boot the system in Firewire Target Disk Mode and browse the files for a version. You might find a text string in something in the configuration file /System/Library/Coreservices/SystemVersion.plist.

MacTracker is a free program that can also list the default OS that came with various Macs, if you absolutely have to fall back to what it was last supporting. It's a wonderful reference for everything from types of RAM systems use to what ports they have and can support; it should list the minimum (and maximum) OS X versions your Mac supports.