My HD needs ground ...

[Amiga4k]

Pin 1 is +5, Pin 4 is +12, with 2/3 as ground. Most PS have separate grounds. Reading across two + leads was interesting, but not beneficial.
As the IDC50 SCSI connector has 7 electrical grounds (aside from 18 electronic grounds), your "guy" might have used the ground within the ribbon cable as electrical ground. (The standard 2.5 drives use power thru the cable, right?).

Adding a ground will not hurt, but I surmise your real problem  is in the cabling. Attaching an extension, while reducing from 3.5" to 2.5" cabling has got to produce stress. Especially after years of wear, jiggling about, vacuuming, etc.

[Amiga4k]

Zac67,
If you do some research:
(1) 2.5" drives are 5 volt, and power is supplied thru the ribbon cable. Both + and -.
(2) you'll find that the commercial adapter for 2.5" to 3.5" consist of either an adapter module, or a modified flat cable. All have in common a 2 lead power plug. (No ground).

ptek,
Like I said, it's the flat cable.Somewhere in the cable it's developed a problem, and failed. Add a ground lead, and by-pass the problem.

Personally, I'd relieve the problem by adding a separate power supply for the HD. The A1200 23 watt PS was never meant to support add-ons beyond a limited range.