I found the following in a text file, and I would like to use the Apple Service Diagnostic Application from a bootable USB key but I cannot find where to download it or set it up? Also is this free software or does it require a separate licence? It sounds like it would be a useful tool for diagnosing Mac problems.

The Apple Service Diagnostic application is designed to run both EFI and Mac OS X tests from an external USB hard drive. Apple Service Diagnostic (EFI) runs low-level tests of the hardware directly and does not require Mac OS X, while Apple Service Diagnostic (OS) uses Mac OS X to run tests. Booting and Using the Apple Service Diagnostic Application - Before using Apple Service Diagnostic, disconnect any Ethernet network, USB, and audio cables. - With the USB hard drive containing ASD 3S123 plugged into a USB port, restart the computer and hold down the option key as the computer boots up into the Startup Manager. To run ASD (EFI) select the "ASD EFI 3S123" drive icon and press return or select it with a mouse click. To run ASD (OS) select the "ASD OS 3S123" drive icon and press return or select it with a mouse click. ASD (EFI) will load in 20-30 seconds; ASD (OS) will load in 2-3 minutes. - After running ASD (OS) or ASD (EFI), press the Restart button to restart the computer back into the normal startup volume, or hold down the option key to get back to the Startup Manager. ASD is no longer delivered as an image to be restored onto a DVD. ASD 3S117 and newer versions requires installation onto an external USB hard drive. For more information, please refer to the document "Installing ASD on a USB hard drive".