It’s probably dead. If not the procedure is to open it, disconnect the battery, wash the PCB and keyboard area with 99% isopropyl alcohol and a toothbrush (lightly) until completely clean, air dry, reconnect battery, pray.

It’s odd, but still not likely to be the power supply. Did you try different cords and sata connector arrangements so you know it is due to the count of drives and not just the last connector (you probably already said this above)?

My MB has a bunch of conditionals in the manual where if slot X is in use slot Y won’t work. Maybe getting all 6 SATA connections working on yours requires that something else NOT be present (M2 SSD, etc).

Good luck

It sounds like a MB limitation. You could get a PCIe SATA extension card, $23 on Amazon. It will be slower than SATA of course.

This sounds more like a motherboard issue than power supply. All of those drives together only pull 25 - 50 W. Removing one drive is a tiny change in power demand. If it were power the drives would work when the CPU is mostly idle and then failures would happen, mostly in the CPU, under load.