Thank you once again for such speedy and detailed advice.
What you've said all makes perfect sense.
Having googled Transient Voltage Suppression Diode I'm beginning to appreciate what these things do. I'm thankful such things exist on these PCBs!
Sorry to drag this out...some more questions arise (prior to me getting assistance from my multimeter mate) - with apologies if any of these repeat other posts elsewhere on forum:
1) Assuming fault is found with one or both TVSs, what's the recommended procedure for removing it/them?
2) How big a risk am I taking powering the HDD up if I've removed one or both TVSs?
3) Regarding choice of (temporary) power source - assuming faulty TVS has been identified and removed - I was considering connecting the HDD back up to its original power supply in the external enclosure since this seems on the face of it to still be functional. Would I be best off avoiding this and using PC Tower instead, or is there a way of verifying the original power supply - perhaps with a multimeter?

- checking for 5V and 12V at the correct amperage at the relevant points? It's more convenient for me to use the original supply if possible since PC Tower is not my own.
4) Is it generally a fairly straightfoward task to source a replacement TVS component and install it on the PCB?
Once again all advice gratefully received.