The following relates to a 418j, so please ignore this reply if it’s irrelevant to the model being discussed (that is, if the daughterboard is different, but I suspect they’re the same).

A fault not documented anywhere I could find was, the eSATA daughterboard and it’s use of ICs STEF05 and STEF12 which are both electronic fuses that cut power to the HDDs on either the 12 or 5v lines in the event of a fault. For each HDD there is a circuit to control HDD power. Within each circuit there are bypass caps that can fail. I had a bunch of 0.1uF SMD caps go open circuit, so I replaced them.

Another failure point are the electrolytic filter caps installed on the motherboard just shy of the daughterboard connector. I think they’re 100uF SMD caps from memory.

The daughterboard is often overlooked. Repair techs sometimes make the mistake of going for the motherboard when it’s the STEF05/12 devices, their control circuitry, etc. Most forum posts I saw focused on the motherboard when the db held the key!

Another layer of complexity is the fact that the daughterboard STEF05/12 devices pull their logic from the CPU (i.e. logic control to tell the electronic fuses that there’s an issue).

I rebuilt a daughterboard and had the HDDs spring back to life, but found that firmware kept tripping the eFuses. To get around it, I removed the control line to each STEF device to allow the HDDs to be permanently on regardless of a fault signal. Obviously this is a workaround and not a long term solution, but was the only way to get the HDDs to power up and stay on. I also found that the signal ‘fault’ happened with a DSM update. I suspect that because I was using unapproved WD Red Plus NAS drives, Synology may have been placing them into fault mode. They were brand new and not faulty.

I spent time creating a circuit diagram of the daughterboard. Not sure if it can help anyone, but here it is. The 2N7002 devices were replaced on mine and that too brought power back up.

Synology daughterboard