My new IDE to SATA adapter came yesterday, so I immediately set about attempting to install the original 500GB drive I bought (which I've since been using as NAS) and getting my old first generation Apple TV to boot. Unfortunately it still failed, so after throwing the word cunt around for a while I had a closer look at the hard drives I'd been using; a WD Black 500GB 7200RPM drive with 32MB of cache (WD5000LPLX) and a WD Blue 500GB 5400RPM drive with 16mb of cache (WD5000LPCX).
Then I had a brain wave. One of the few instances where I'd read about someone succeeding in upgrading their ATV with a SATA adapter and a big (in their case 1TB) drive, they had used a 5400RPM drive with 8MB of cache. As a last ditch effort to try and solve this annoying issue I bought a third and final drive (because if this didn't work I was giving up for good); a WD Blue 5400RPM 750GB drive with 8MB cache (WD7500BPVX) and it only fucking works!
The two previous IDE to SATA adapters I used may have also played a part in causing me grief, but third time is apparently the charm as the third (and again final) adapter I bought was used by several others who claimed success in upgrading their ATV; an Advantech model PCM-233B-00A1E. It's considerably more expensive than the other adapters I've used, mostly because I had to buy it from somewhere other than Advantech as they apparently won't sell to consumers outside the US unless you are VAT registered, but it works, so I'm not complaining.
I should also note at this point the adapter has its full assortment of 44 pins on the IDE 2.5" connector where as the blue connector on the ribbon cable technically only has 41, with Pin 20 blanked off (this is normally done to prevent idiots putting the cable in upside-down). But it was easy enough to just poke a hole through the bit of plastic covering where that pin should go. Of course once I'd tested it and made sure it was all working, fitting everything back into the case wasn't exactly easy.
Because the adapter is quite large, I had to use a SATA power and data extension cable as well (I used an 8"/20cm cable but a 6" cable might have been a better bet). This allowed me to have the adapter sat over the WiFi card in the gap between the power supply and the cooling fan, which is pretty much the only place you can put it. I put a bit of insulation tape over the top of the adapter too so it couldn't short out on the bottom case. It took a few goes to get it all in a good position so the bottom case could be screwed back on but I managed it, all be it with a slight bulge on the case, but I didn't do the screws up particularly tight and once the rubber base was back on it doesn't notice too much.
As for how I eventually went about reinstalling the software onto the new drive, well there's plenty of tutorials on doing it via the Terminal on a Mac which I tried, but eventually I came across this page which explains (vaguely) how to do it via a USB stick and it is just so much easier and quicker. There are two image files, one with v2 of the original Apple TV OS and one with v3. I did it on Windows using an application called USB Image Tool and a Sandisk Ultra USB Stick I had lying around, but you can do it using a Mac or on Linux too by following this guide which has a more detailed tutorial for all methods.
They don't use USB-IT for their Windows tutorial but I still recommend using USB-IT as it's small and portable. Anyway I tried the v3 image initially but the ATV failed to do a factory restore, so I tried v2 and it finally worked. It just meant I had to let the ATV update itself once it had restarted (which for now still works, but if Apple will ever do away with those updates in the future is anyone's guess). This is apparently a common problem when using this method to restore, though I suspect it is to do with what version of the OS your ATV shipped with. Mine when I got it had the older v2 software as v3 hadn't been released at the time. Perhaps if your ATV came with v3 then the v3 image will work fine for you.
Anyway, to close this once and for all if you're thinking about doing this upgrade then I suggest you invest in one of those Advantech adapters and make sure your hard drive only has 8mb of cache, it might also make a difference if it's only 5400RPM too. I'm not guaranteeing that every drive you use will work provided it's 5400RPM with 8MB of cache but it certainly seemed to make the difference for me. And after so many years of people asking the question of what hard drive does the Apple TV support, it'd be nice for me if I've potentially stumbled on the answer.
I mean sure it's hard to believe I'm the only one who has, but if there are others out there that knew this I couldn't find that helpful article or forum post where they explained it, so fuck them, I'm taking all the credit here! Then again it is 2017, the first generation Apple TV was released over ten years ago and has essentially been discontinued for seven of them and with support for it apparently silently dropped in iTunes 12.7 (unless that's just a bug), the usefulness of finding out now what HDD works in the damn thing is somewhat of a moot point.