Faulty HDD or PSU?

A quote from Mike 99 last October to a similar question. My home build PC has been fine for the last 4 years and without any problems that I didn't cause myself. However, the last few months has seen problems arising where my BIOS does not always recognise my single HDD, therefore I cannot boot. My quickie-fix has been as Mike suggested and most of the time it works fine. However, the last few days the problem seems to have taken on a permanent refusal to recognise my HDD at all. (I know, I know - I should have changed the PSU then and there!)

Everything else but the HDD works or is recognised (DVD drive, CPU fan, power light, etc). The HDD itself spins up fine and no ticking to give me that ominous warning of platter smash. I've swapped power and data cables and plugs about endless times, cleaned and prayed even more, reseated everything that's reseatable, measured voltages (12v is going to the HDD on the occasions the PSU works okay), fiddled and reset the BIOS in all sorts of permutations but sadly with no joy.

So, faulty PSU as Mike suggests above or have I left it to late and blown the HDD circuitry. Or both? I've no reference system to try either component on and my local computer shop wants more than the cost of a new PSU just to look at it (seeing that its a "home build" ). To be clear, this is not a RAID configuration just a single bootable hard disk.

Details: Gigabyte GA-K8N Pro-Sli mobo, Gigabyte GeForce 6600GT 256MB GPU, 4 x 1GB Crucial PC3200 RAM, AMD Athlon64 3200+ CPU, 250Gb Barracuda 7200.9 Sata2 HDD, Enermax Liberty 400W PSU, Win XP SP3 OS.

Comments and suggestions to solve much appreciated.

Jake