Question seagate hdd 7200rpm gets enough power to run only through molded power sata cable connector

I have used extensions and splitters before to get enough length or more drives than a PSU had cable length or connectors for, but in this case I think you should just bite the bullet and buy a better PSU with enough, quality, connectors rather than throwing money at the deficiencies of a junky PSU that may not even have a long life ahead of it.

However, yes it does seem like your insulation displacement SATA connector (aka crimped) is poorly made and causing problems. You could try opening it, moving the connector a little further up the wire, cutting off excess wire and see how that does, but it could be that this cheaply built PSU has a smaller gauge wire than the connector was meant for and this is contributing to the problem because even if the connector was poorly attached, if it is latched shut then it shouldn't be letting the wires wiggle around in it, suggesting that the wire diameter is too small for a good result.

There are hacks you could try to make the wire fit tighter, like put some hot glue in right before you snap the connector shut (though, the latches on the connector might break off if you even try to open it in the first place), or epoxy, etc, though often the connector needs shut before it pushes the wires down into the insulation displacement contacts so anything added before the connector is shut again, could interfere with that and also make a mess. I would only try this if desperate, not as a long term solution.

Another option if you have other junky /scrap PSU lying around, is take the SATA leads off another PSU and splice/solder then onto your PSU instead of the insulation displacement connector. I would sooner do that, rather than mess around with an ill-fitting insulation displacement connector.

You could try adding a splitter instead, and it can't be much worse than what you have now, but I would try not to put any more HDDs on the same run of cable from the PSU than you have to, especially not the full 3.5" 7K2 HDDs since they draw a lot more (12V) power than the laptop drive or an SSD. If the cables have undersized wire gauge, and possibly also poor fitting generic connectors, then you could still face poor power delivery.

If you really can't budget for a better PSU, then at least a splitter isn't expensive to try, but I would be careful about what data you have on the HDDs and make backups of that data in case there is a situation where bad power causes data loss on them.

Personally, I would look at replacing that PSU. Messing around with these connectors and such is just opening yourself up to liability if there was ever a problem such as a fire. It isn't worth it for a $50 part to be potentially held liable for a fire....