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If you build those power cable, pls use spare disk to test before put in production. Sometimes we will make mistake and burn all stuff. I have that and burn 4 of Blray drive, lucky they all cheap and I have fix it.

It likely Seagate disk with LSI HBA, wakeup / power-save issue, most read error occur during disk wakeup.

Could you try two test 1. If only insert 1 NVMe ( you have two ), try swap M2 slot. 2. If have another brand M2, pls try mix different model.

How about preclear status, still positive ?

Yes, those are capacitor, use to minimise ripper voltage, it also share the loading of PSU inside capacitor, it should little bit help to extend the PSU life.

I will build something use DIY component. One side coming from PSU and the other side connect to harddisk. 4 terminal are 12v G G 5V. Each "Insulated Terminal Barrier Strip" ( red black yellow ) will insert 3 or 4 wire and I will soldering to ensure reliable connection. I will build different harddisk wire set ( black wire with correct diameter ) for different need, just detach / attach at terminal block. Each wire will connect max 4 disk, and inset to DIY SATA plug at the end. Anyway, max 16 disks, never use Y power splitter. Or something like that, if connect point have two "Insulated Terminal Barrier Strip", then it can increase the no. of connect device or to change the direction.

May be try eliminate the Y power splitter and try again to verify problem reproduce or not first.

Pls source different supplier.

Sure. All, black is ground red black is 5v yellow black is 12v

With dual connector, it can increase the power handling, you will found gaming GPU card, power socket increase when power draw increase. Dual connector should connect individual power wire/path. Yes, measure DC voltage, perform some measurement still better then nothing, if you don't family with this then pls ignore it. Voltage measurement will provide some basic info., ripple voltage also a key factor.

If you compare the reverse cable with forward cable and haven't found signal data pair cross ( TX RX pair, ignore +- ), then cable must wrong. It could be wrong product send to you even it lables reverse, in fact, you can't identify by the product outlook. Do you have any HBA which use SFF8087 ? Then you can use it to test the cable.

Then the reverse cable actually is forward cable. You will got different observation ( data signal pair cross ) with forward / reverse cable.

If they connected well and reliable then no problem, but Y splitter should avoid due to have chance of voltage drop in heavy load. 750w far enough ( if no monster GPU ), but try another PSU also a good practice.

I would suggest you buy another reverse cable and try. Do you have another reverse cable ?? If haven't : Seems you have electronic knowledge, you can plug the forward cable in SFF8087, check the SATA plug port 1 either data pin short to blackplane SATA_1 port which data pin, then do the same thing with reverse cable, if both pin also in short then the reverse cable would be problem. ** edit : due to capacitor in the path, you must test ahead the capacitor, I hope you understand what I mean and the test purpose **

Then reverse cable correct and you connect to mobo/controller in SATA.