I got everything connected up ok - and I noticed even that the loose PCI cable could only go one way - which is handy (i.e. connector blocks are slightly different either end).
They Are keyed differently to ensure they can only put inserted one way and because some motherboards use a 6 or 8 pin plug now to supply more power to the CPU. This way there is now way to use the wrong power supply lead on the wrong component
Regarding your Data Drive. Open the case up and ensure you have not knocked out any cable running from the drives to the MB or their power leads. This can easily be done will plying about in the case, we have all done it :-)
I can not tell much about the drives you have from the pics. They look like SATA ones from the MB headers but we would need a pic of them to be sure.
Since your PC is booting fine by the looks of things you can try another thing.
Go to start menu and right click on my computer. Select Manage from the list. Then select disk management. The naming may differ depending on your OS, XP / Vista / 7 etc but they are very similar.
Here you can see all Volumes that are visible to your OS. These can include USB drives and HDD, etc.
Picture borrowed from another site.....
Does your data drive show up here?
I turned things on and heard a slight, but frightening electric 'click', like static. In trepidation I carried on and installed the card - which works great and is silent. BUT: my D: drive is not accessible and it's where ALL my data is. I keep programs on C:. So, I assume I've blown whatever makes the D: disk live. I really need some big time help now.... sorry....
What sequence did you install in mate. Having the PC disconnected from the mains and removing all power from the Capacitors, as described above by hitting the power button again, is the safest way to install components.
Anything else and you could risk damaging components.
In all rights you should ground yourself, statically I mean, before carrying out any work too.
What shall I check to diagnose exactly what's happened? How can I then fix it? Did the graphics card overload the PSU?
PSU's are getting quite smart now. If the load that the computer is trying to draw is above the maximum load that the PSU is rated for, in your case 600W, it will shut things down. You only get PSU issues using poor quality PSU's. You have a good quality one so no need to worry.
Also if you used a power calculator, PSU Calculator, available online you can work out roughly how much power your system could pull in theory. For 90% of all PC's a 500-600W PSU is more than adequate, I believe the average is as low as 350-400W load max. Only once you add SLI / Crossfire and 10+ HDD etc will you start to need the larger capacity PSU's.
Ed