I was poking around our GSX account looking for AST updates, when I came across ASD. Thinking they were the same just they changed the name (I'm about as n00bish at the GSX site as you can get), I downloaded the latest version. Upon discovering this ASD thing needed to be on a USB stick / HD i proceeded to install on there.
Since I just had gotten a machine that needed to have a hardware test run I ran it on there, but it said it wasn't compatible. Do I need to load a specific version for each machine? I ended up downloading one for that specific machine which worked, but that seems to be a bit of a PITA to do for every hardware model we have.
What's the point of AST (which said that the machine passed), but then the OS version of the ASD found a failure...
They are both suites of tests. Short answer: Use AST (by which I mean MRI) to verify everything is plugged in. Save ASD for when you're really stumped about why something is busted or to completely rule out hardware failure in troubleshooting.
Long Answer: AST most notably includes MRI, which is basically a roll call and quick test for available hardware, basically "is everything plugged in?" It also includes things like interactive keyboard and trackpad tests, battery tests, and cooling systems diagnostics. AST also hooks into GSX as it is a net-booted test environment, so it may need to be run to qualify a repair in-warranty (which is why you see the people at the Genius bar run it at Apple stores during appointments). ASD on the other hand runs in two variants, EFI or OS and yes, needs to be loaded on a hard drive or thumb drive, though you can partition a drive many times and pre-load all the model-specific tests onto different partitions. EFI tests lower level hardware, OS can test all kinds of things, like sleep/wake, graphics, etc. Keep in mind no test is perfect and both AST and ASD can have false positives. For a full list of the ASD version you need, you can search "ASD" in the big search bar on GSX and look for the marketing name - like MacBook Pro ( 15-inch, Late 2011) - of the model you're looking for and it'll give you a download link for the appropriate version. There's extension documentation on AST too if you wanna know what MRI checks.
AST was designed to be a customer-facing utility for AASPs. Techs are required to use it and like said, it does a quick check of a given component.
ASD is what I tend to trust more. I can set it in looping mode and deal with intermittent issues and it thoroughly can check an individual component (say the logic board) for individual smaller failures.
I use them in tandem myself...I check in with AST, do my work, make sure it passes ASD and then use AST again for any reason Apple calls for it (say OS tests or required specific tests for an issue.) Once it passes both or is within Apple spec, only then do I consider a repair done.