I am seeking to find a Macbook Air serial number from within the backup files created by the Mountain Lion operating system and stored on an external hard drive. Exactly which file do I need to be looking at? I made the presumption that the backup created by time machine would store the uuid to ensure that the physical MAC address (7 layer OSI model) would be the one that received the backed up files of the MacBook Air. Perhaps in Extended Firmware Interface (EFI) folder although I did see a file mentioned; in a post elsewhere which was com.apple.TimeMachine.MachineID.plist. The same post also said that a sparse bundle created by time machine contained the hardware UUID (effectively the serial number?).
The UUID is not the same as the serial number.
When you use any Mac, some machine-specific preference files are created in your Library/Preferences/ByHost/ directory. The filenames are of the form
So, if you really want the UUID of a previously used machine, you can probably dig it up from some backups in that way. I don't know how useful that would be though.
I think TimeMachine stores UUIDs of the filesystem, but not the computer itself. Checking the logs on one of our servers here, I see several fsUUID and eventDBUUID but nothing for the machine.
It could be written to the disk if you had previously run the system profiler. This is the same as "About this Mac" except information is dumped into the Terminal. You could save the report thusly:
system_profiler > ~/systeminfo.txt
Otherwise the serial as mentioned is read directly from the logic board and generally not logged to disk.